C2_Chapter6_HVAC II_Air Systems PDF

An Erasmus Mundus International Joint Master Degree High Performance of HVAC systems 6. HVAC II: Air conditioning systems PhD Álvaro Campos Celador PhD Iker González Pino PhD Pello Larrinaga Alonso Department of Energetic Engineering University of the Basque Country MSc in Smart Cities and Commun...

An Erasmus Mundus International Joint Master Degree High Performance of HVAC systems 6. HVAC II: Air conditioning systems PhD Álvaro Campos Celador PhD Iker González Pino PhD Pello Larrinaga Alonso Department of Energetic Engineering University of the Basque Country MSc in Smart Cities and Communities  6. HVAC II: Air Conditioning Systems 1. Introduction to air conditioning systems 2. Air Handling Units (AHU) 3. Air distribution systems 4. Terminal units for air conditioning systems 5. Condensing loops 6. Control of air conditioning systems Table of contents 3  6. HVAC II: Air Conditioning Systems 1. Introduction to air conditioning systems 2. Air Handling Units (AHU) 3. Air distribution systems 4. Terminal units for air conditioning systems 5. Condensing loops 6. Control of air conditioning systems Table of contents 4  1. Introduction to air conditioning systems  Air systems provide complete sensible and latent cooling, heating and (de)humidification capacity in the air supply.  Heating may be accomplished either by the same airstream or by a separate heater.  Two basic mechanisms to vary the energy removed/supplied by the supply air: Vary the temperature of the supply air (Constant Air Volume - CAV). Vary the amount of the supply air (Variable Air Volume - VAV). AHU AHU Constant Air Volume unit (left) and Variable Air Volume unit (right) High Performance of HVAC systems 5 Department of Thermal Engineering  1. Introduction to air conditioning systems INITIAL CONSIDERATIONS All-air systems operate by maintaining a difference between supply air temperature and desired room temperature (T). Any load that affects this differential must be considered:  All fans convert shaft power into heat. Inefficiencies are added load if the motor is in the airstream. Whether the fan is blow-through or draw-through affects how this load must be accounted for.  The supply duct may gain or lose heat from the surroundings. Uninsulated cooling delivery ducts are subject to condensation: water damage, mold growth…  Controlling humidity in a space can affect the air quantity and become the controlling factor. High Performance of HVAC systems 6 Department of Thermal Engineering  1. Introduction to air conditioning systems High Performance of HVAC systems 7 Department of Thermal Engineering  1. Introduction to air conditioning systems AIR TEMPERATURE VERSUS AIR QUANTITY  Designers have considerable flexibility in selecting supply air temperature and corresponding air quantity.  The relationship of the T and air volume is approximately linear and inverse.  The traditional all-air system is typically designed to deliver air as low as 13 ºC supply air: Desired indoor temperature or approximately 24 ºC. Modest latent heat loads and low air absolute moisture.  Lower supply air temperatures may be required in spaces with high latent loads. High Performance of HVAC systems 8 Department of Thermal Engineering  1. Introduction to air conditioning systems AIR TEMPERATURE VERSUS AIR QUANTITY  In cold-air systems the supply temperature is designed as low as 7 ºC: smaller ducts and fans.  The initial cost of lower airflow and low air temperature must be calculated against potential problems of distribution, condensation, air movement, and decreased removal of odors and gaseous or particulate contaminants.  Advantages of cold-air systems include: Lower humidity levels. Reduced fan energy consumption. High Performance of HVAC systems 9 Department of Thermal Engineering  1. Introduction to air conditioning systems  All air systems are used in buildings of all sizes that require individual control of multiple zones: office buildings, schools and universities, laboratories, hospitals, stores, hotels… High Performance of HVAC systems 10 Department of Thermal Engineering  1. Introduction to air conditioning systems ADVANTAGES DISADVANTAGES Major equipment in an unoccupied area Ducts installed in ceiling plenums require additional clearance Noise-producing equipment away from the occupied Larger floor plans to allow adequate space for vertical area shafts Great potential to use outdoor air for economizer Transport energy larger than in other systems cooling Simple seasonal changeover Equipment rooms represent nonrentable spaces Wide choice of zoning, flexibility, and humidity control More difficult accessibility to terminal devices, Heat recovery may be readily incorporated dampers, etc. Failure of a central component affects all zones served Flexible designs and adaptability to varying local requirements Suitability to applications requiring unusual ventilation Close operating conditions if high quality controls are used Longer lifetime and less O&M costs than many terminal systems High Performance of HVAC systems 11 Department of Thermal Engineering  1. Introduction to air conditioning systems ZONING  Exterior zones are affected by weather conditions and may require both heating and cooling at different times.  The need for separate perimeter zone heating is determined by: Severity of heating load Nature and orientation of building envelope Effects of downdraft at windows and radiant effect of cold glass surfaces Type of occupancy Operating costs  Separate perimeter heating can operate with any all-air system: greatest application with VAV systems for cooling. High Performance of HVAC systems 12 Department of Thermal Engineering  1. Introduction to air conditioning systems ZONING  Interior spaces have relatively constant conditions because they are isolated from external influences.  Interior zones usually require cooling throughout the year.  A VAV system has limited energy advantages for interior spaces, but provides simple temperature control.  Interior spaces with a roof exposure may require treatment similar to perimeter spaces. High Performance of HVAC systems 13 Department of Thermal Engineering  1. Introduction to air conditioning systems ZONING High Performance of HVAC systems 14 Department of Thermal Engineering  1. Introduction to air conditioning systems ZONING High Performance of HVAC systems 15 Department of Thermal Engineering  1. Introduction to air conditioning systems  The air distribution system of an all-air system consist of two major subsystem: Air-handling units that generate conditioned air under sufficient positive pressure to circulate it to and from the conditions spaces. A distribution system systems that only carries air from the air-handling unit to the spaces being conditioned. High Performance of HVAC systems 16 Department of Thermal Engineering  6. HVAC II: Air Conditioning Systems 1. Introduction to air conditioning systems 2. Air Handling Units (AHU) 3. Air distribution systems 4. Terminal units for air conditioning systems 5. Condensing loops 6. Control of air conditioning systems Table of contents 17  2. Air Handling Units (AHU) Air Handling Units (AHU) are used in big installations to maintain air conditions (temperature, humidity, air quality) within desired ranges. https://youtu.be/KCiv8IAUkh8 An AHU is responsible for treating air and propelling it to the zones to be conditioned. AHU are made of a series of elements in order to carry out the various treatments air requires before being emitted to the conditioned zones. AHU do not produce thermal energy; they receive it from heating and/or Air Handling Unit cooling generators. High Performance of HVAC systems 18 Department of Thermal Engineering  2. Air Handling Units (AHU) High Performance of HVAC systems 19 Department of Thermal Engineering  2. Air Handling Units (AHU) High Performance of HVAC systems 20 Department of Thermal Engineering  2. Air Handling Units (AHU)  The basic air-handling system is an all-air, single-zone HVAC system.  The air-handling unit may be designed to supply constant or variable air volume.  Normally, the equipment is located outside the conditioned area.  The equipment can be adjacent to the primary heating and refrigeration equipment or at considerable distance. High Performance of HVAC systems 21 Department of Thermal Engineering  2. Air Handling Units (AHU) PRIMARY EQUIPMENT COOLING & HEATING  Either central or local equipment can provide cooling.  Most large systems with multiple central air-handling units use a central refrigeration plant.  Small, individual air-handling equipment can: be supplied with chilled water from central chillers use direct-expansion cooling with a central condensing be air cooled and totally self-contained  Usually a central fuel-fired plant is more desirable for heating large facilities. High Performance of HVAC systems 22 Department of Thermal Engineering  2. Air Handling Units (AHU) AIR-HANDLING EQUIPMENT  Packaged air-handling equipment is commercially available in many sizes, capacities, and configurations: suitable for small and large buildings.  In large systems (over 25 m3/s) air-handling equipment is usually custom-designed and fabricated to suit a particular application.  The designer must properly determine an AHU’s required: Supply air temperature and volume Humidification and dehumidification capacities Outdoor air requirements Return, relief and exhaust air volume requirements Desired space pressures Filtration Heating and cooling coil capacities Required pressure capabilities of the fan(s) High Performance of HVAC systems 23 Department of Thermal Engineering  2. Air Handling Units (AHU) MECHANICAL EQUIPMENT ROOMS (MERs) The type of facility and other factors will determine where the air-handling equipment is located. High Performance of HVAC systems 24 Department of Thermal Engineering  2. Air Handling Units (AHU) MECHANICAL EQUIPMENT ROOMS (MERs) One important design decision: central or decentralized MER? Decentralized MER Central MER High Performance of HVAC systems 25 Department of Thermal Engineering  2. Air Handling Units (AHU) MECHANICAL EQUIPMENT ROOMS (MERs) CENTRAL MER DECENTRALIZED MER Fewer total pieces of equipment to maintain Reduced size of ducts: less space required for ductwork Maintenance is concentrated at one location Reduced equipment size: cheaper and less sophistication Filtration is easily enhanced Possibility of turning off in unoccupied areas Energy recovery opportunities may be more practical Failure of an AHU affects only part of the building Vibration and noise control and equipment more simple to handle High Performance of HVAC systems 26 Department of Thermal Engineering  2. Air Handling Units (AHU) AIR-HANDLING UNIT PSYCHROMETRIC PROCESSES High Performance of HVAC systems 27 Department of Thermal Engineering  2. Air Handling Units (AHU) AIR-HANDLING UNIT PSYCHROMETRIC PROCESSES The basic methods used for cooling and dehumidification include the following:  Direct expansion (refrigerant) takes advantage of the latent heat of the refrigerant fluid.  Chilled-water (fluid-filled) coils use temperature differences between the fluid and air to exchange energy.  Direct spray of water in the airstream, an adiabatic process, uses the latent heat of evaporation of water to reduce dry-bulb temperature while increasing moisture content.  In the wetted duct or supersaturated system tiny droplets of free moisture are carried out by the air into the conditioned space, where they evaporate and provide additional cooling.  Indirect evaporation adiabatically cools outdoor or exhaust air from the conditioned space by spraying water, then passes that cooled air through one side of a heat exchanger, passing air to be supplied through the other side of the heat exchanger. High Performance of HVAC systems 28 Department of Thermal Engineering  2. Air Handling Units (AHU) AIR-HANDLING UNIT PSYCHROMETRIC PROCESSES Direct-expansion or chilled-water cooling and dehumidification (left), Direct spray of water in airstream cooling (middle) and Supersaturated evaporative cooling (right) High Performance of HVAC systems 29 Department of Thermal Engineering  2. Air Handling Units (AHU) AIR-HANDLING UNIT PSYCHROMETRIC PROCESSES The basic methods used for heating include the following:  Steam uses the latent heat of the fluid.  Hot-water (fluid-filled) coils use temperature differences between the warm fluid and cooler air.  Electric heat also uses the temperature difference between the heating coil and the air to exchange energy.  Direct or indirect gas or oil-fired heat exchangers can also be used to add sensible heat to the airstream. Steam, hot-water, and electric heating, and direct and indirect gas and oil-fired heat exchangers High Performance of HVAC systems 30 Department of Thermal Engineering  2. Air Handling Units (AHU) AIR-HANDLING UNIT PSYCHROMETRIC PROCESSES The basic methods used for humidification include the following:  Direct spray of recirculated water into the air stream (air washer) reduces the dry-bulb temperature while maintaining an almost constant wet bulb, in an adiabatic process. The air may also be cooled and dehumidified, or heated and humidified, by changing the spray water temperature.  Compressed air that forces water through a nozzle into the air-stream is essentially a constant wet-bulb (adiabatic) process.  Steam injection is a constant dry-bulb process. As the steam injected becomes superheated, the leaving dry-bulb temperature increases. High Performance of HVAC systems 31 Department of Thermal Engineering  2. Air Handling Units (AHU) AIR-HANDLING UNIT PSYCHROMETRIC PROCESSES Direct spray humidification (left), Steam injection humidification (middle) and Chemical dehumidification (right) High Performance of HVAC systems 32 Department of Thermal Engineering  2. Air Handling Units (AHU) AIR-HANDLING UNIT PSYCHROMETRIC PROCESSES The basic methods used for dehumidification include the following:  Air can be dehumidified if a fluid with a temperature below the airstream dew point is sprayed into the airstream. The moisture condensed from the airstream condenses on, and dissolves in, the spray drop-lets.  Adiabatic mixing of two or more airstreams into a common airstream.  Chemical dehumidification involves either passing air over a solid desiccant or spraying the air with a solution of desiccant and water. Both of these processes add heat (latent heat of wetting) to the air. Usually 465 kJ/kg of moisture is removed. Adiabatic mixing of two airstreams High Performance of HVAC systems 33 Department of Thermal Engineering  2. Air Handling Units (AHU) MAIN COMPONENTS OF AN AHU https://youtu.be/KCiv8IAUkh8 AHU can be made of the following sections: FANS are responsible for supplying air. COOLING OR HEATING COILS cool down or heat up air to be treated. FILTERS eliminate undesired particles of air and ensure its quality. HUMIDITY SECTIONS adapt the humidity degree of air. MIXING SECTIONS control the amount of exhaust air and clean outside air that is introduced in the ATU. RECOVERY SYSTEMS allow recovering part of the energy content of the return air which is used to preheat the supply air, in case it is necessary. High Performance of HVAC systems 34 Department of Thermal Engineering  2. Air Handling Units (AHU) MAIN COMPONENTS OF AN AHU Exhaust air Outside air Supply-air Humidifier fan Return air Supply air Filter Exhaust- Air-mixing Cooling coil Heating coil return fan chamber Parts of an AHU High Performance of HVAC systems 35 Department of Thermal Engineering  2. Air Handling Units (AHU) MAIN COMPONENTS OF AN AHU 1. FANS  This section is responsible for propelling air to be conditioned through the different sections of the AHU.  When high ventilation rates are required, two fans sections are used: forward section and backward section. Fans High Performance of HVAC systems 36 Department of Thermal Engineering  2. Air Handling Units (AHU) MAIN COMPONENTS OF AN AHU 1. FANS  The return air fan is optional on small systems, but essential for proper operation of air economizer systems for free cooling from outdoor air if the return path has a significant pressure drop (>75 Pa): Provides a positive return and exhaust from the conditioned space. Ensures that the proper volume of air returns from the conditioned space. Prevents excess building pressure when economizer cycles introduce more than the minimum quantity of outdoor air Reduces the static pressure against which the supply fan must work.  A relief (or exhaust) air fan relieves ventilation air introduced during air economizer operation and operates on when this control cycle is in effect. High Performance of HVAC systems 37 Department of Thermal Engineering  2. Air Handling Units (AHU) MAIN COMPONENTS OF AN AHU 1. FANS  Two placements of the supply air fan section are common: Downstream of the cooling coil (draw-through): usually provides a more even air distribution over all parts of the coil. Upstream of the cooling coil (blow-through): the blast effect of the supply fan outlet can concentrate a high percentage of the total air over a small percentage of the coil. All-air draw-thru unit (left) and blow-thru unit (right) High Performance of HVAC systems 38 Department of Thermal Engineering  2. Air Handling Units (AHU) MAIN COMPONENTS OF AN AHU 1. FANS  A fan uses a power-driven rotating impeller to move air: kinetic energy is imparted to air.  According to the direction of airflow through the impeller, fans are generally classified as: Centrifugal Axial Mixed Cross flow  A fan produces pressure and/or airflow because the rotating blades of the impeller impart kinetic energy to the air by changing its velocity: tangential and radial velocity in centrifugal fans and axial and tangential velocity in axial fans. High Performance of HVAC systems 39 Department of Thermal Engineering  2. Air Handling Units (AHU) MAIN COMPONENTS OF AN AHU 1. FANS  Centrifugal fan impellers produce pressure from the centrifugal force created by rotation and from the kinetic energy imparted to the air.  The velocity is a combination of rotational velocity of the impeller and airspeed relative to the impeller.  Blades can be inclined forward or backward. Backward-curved blade fans are generally more efficient. Centrifugal fan components High Performance of HVAC systems 40 Department of Thermal Engineering  2. Air Handling Units (AHU) MAIN COMPONENTS OF AN AHU 1. FANS  Axial-flow fan impellers produce pressure principally by the change in air velocity as it passes through the blades.  These fans are divided into three types: Propeller fans usually have a small hub-to-tip ratio impeller mounted in an orifice plate or inlet ring. Tubeaxial fans usually have reduced tip clearance and operate at higher tip speed. Vaneaxial fans are tubeaxial fans with guide vanes and reduced running blade tip clearance: improved pressure, efficiency and noise characteristics. Axial fan components High Performance of HVAC systems 41 Department of Thermal Engineering  2. Air Handling Units (AHU) Types of axial fans Types of centrifugal fans High Performance of HVAC systems 42 Department of Thermal Engineering  2. Air Handling Units (AHU) MAIN COMPONENTS OF AN AHU 1. FANS  The efficiency of a fan is the ratio between the power transferred to the air flow and the power used by the fan: 𝜌 · 𝑉 · 𝑐 · ∆𝑇 η 𝑊  In certain applications, it may be desirable to calculate the temperature rise across the fan: ∆𝑃 ∆𝑇 𝜌·𝑐 ·η If the motor is not in the airstream, the efficiency is the fan total efficiency; if the motor is in the airstream, the efficiency is the set efficiency. High Performance of HVAC systems 43 Department of Thermal Engineering  2. Air Handling Units (AHU) MAIN COMPONENTS OF AN AHU 2. HEAT EXCHANGERS (COILS) Their mission is to heat up or cool down air. Constructively similar to condensers and evaporators. Consist of a copper coil and fins. Two connections for the water/refrigerant circuit and a condensate collection tray (only in cooling coils). HEATING COILS COOLING COILS Heat exchanger High Performance of HVAC systems 44 Department of Thermal Engineering  2. Air Handling Units (AHU) MAIN COMPONENTS OF AN AHU 2. HEAT EXCHANGERS (COILS)  Preheat coils are heating coils placed upstream of a cooling coil. They can use steam, hot water, or electric resistance as a medium. If the percentage of outdoor air is low a preheat coil may be dispensable and/or if building heating is provided elsewhere (e.g., perimeter baseboard, radiators…). A preheat coil should have wide fin spacing, accessible for easy cleaning, and be protected by filters. Hot-water preheat coils should be piped for counterflow so that the coldest air contacts the warmest part of the coil surface first. A constant-flow recirculating pump should be considered if the local climate and anticipated percentage of outdoor air may result in freezing conditions at a hot-water preheat coil. High Performance of HVAC systems 45 Department of Thermal Engineering  2. Air Handling Units (AHU) MAIN COMPONENTS OF AN AHU 2. HEAT EXCHANGERS (COILS)  Cooling coils remove sensible and latent heat from the air. The cooling medium can be either chilled water or refrigerant (DX). Flow circulation of chilled water during freezing weather minimizes coil freezing and eliminates stratification. Antifreeze solutions or complete coil draining can also prevent freezing. The drain pan should be sloped to a drain. Constant presence of moisture in the cooling coil drain pan and nearby casing may require stainless steel in that portion. High Performance of HVAC systems 46 Department of Thermal Engineering  2. Air Handling Units (AHU) MAIN COMPONENTS OF AN AHU 2. HEAT EXCHANGERS (COILS)  Reheat coils are heating coils placed downstream of a cooling coil. Reheat systems are strongly discouraged, unless recovered energy is used. Positive humidity control is required to provide comfort conditions for most occupancies. Reheating may be necessary for applications where temperature and relative humidity must be controlled accurately (laboratory, health care, or similar applications). Heating coils located in the reheat position are frequently used for warm-up. Electric coils may also be used. High Performance of HVAC systems 47 Department of Thermal Engineering  2. Air Handling Units (AHU) MAIN COMPONENTS OF AN AHU 3. PRE-FILTERS AND FILTERS  A system’s overall performance depends heavily on the filter section.  Their mission is to eliminate particles air may sweep along.  Various filters sections if high air quality is required: Thick particles and fibers: wire and felt pre-filters. Pre-filter High Performance of HVAC systems 48 Department of Thermal Engineering  2. Air Handling Units (AHU) MAIN COMPONENTS OF AN AHU 3. PRE-FILTERS AND FILTERS  A system’s overall performance depends heavily on the filter section.  Their mission is to eliminate particles air may sweep along.  Various filters sections if high air quality is required: Thick particles and fibers: wire and felt pre-filters. Finer particles: bags or mesh filter clothes or thicker filtering paper. Filters High Performance of HVAC systems 49 Department of Thermal Engineering  2. Air Handling Units (AHU) MAIN COMPONENTS OF AN AHU 3. PRE-FILTERS AND FILTERS  A system’s overall performance depends heavily on the filter section.  Their mission is to eliminate particles air may sweep along.  Various filters sections if high air quality is required: Thick particles and fibers: wire and felt pre-filters. Finer particles: bags or mesh filter clothes or thicker filtering paper. High efficiency filters have an electrostatic system. Gases dissolved in air: activated carbon filters. Electrostatic filter High Performance of HVAC systems 50 Department of Thermal Engineering  2. Air Handling Units (AHU) MAIN COMPONENTS OF AN AHU 4. HUMIDIFICATION SECTION  Humidifiers may be installed as part of the AHU or/and in terminals at the point use.  Humidifying capacity should not exceed the expected peak load by more than 10%.  If humidity is controlled, a limiting humidistat and fan interlock may be needed.  They are typically placed between a preheat and a cooling coil. High Performance of HVAC systems 51 Department of Thermal Engineering  2. Air Handling Units (AHU) MAIN COMPONENTS OF AN AHU 4. HUMIDIFICATION SECTION  Humidifiers can be broken into two basic categories depending on when the energy is added: Isothermal units use external energy to produce steam, and the process results in a near-constant air temperature. Adiabatic units, called atomizers or evaporative units, allow direct contact between the water and airstream, and the humidification process results in a lower air temperature: all of the energy for the transformation is provided by the airstream. High Performance of HVAC systems 52 Department of Thermal Engineering  2. Air Handling Units (AHU) MAIN COMPONENTS OF AN AHU 4. HUMIDIFICATION SECTION  Three main ways of humidifying: Electrical resistance Felt or mesh Pressured water Evaporative humidifier High Performance of HVAC systems 53 Department of Thermal Engineering  2. Air Handling Units (AHU) MAIN COMPONENTS OF AN AHU 5. MIXING SECTION  An air mixing chamber (box) is installed with two purposes: Air dumper Through out part of the inside air Absorb the same air quantity from the outside  It consists of a chamber with two or three motorized air dampers.  Installed both right after the outside air damper or after the ventilation return (if exists). Air-mixing chamber High Performance of HVAC systems 54 Department of Thermal Engineering  2. Air Handling Units (AHU) MAIN COMPONENTS OF AN AHU 5. MIXING SECTION  Suggestions in order to increase the mixing provided by mixing boxes: The outdoor air damper should be located as close as possible to the return air damper. A higher velocity through the return air damper facilitates air balance and may increase mixing. Positioning dampers so that the return and outdoor airstreams are deflected toward each other. Placing the outdoor air damper above the return air damper increases mixing by density differences. Mixing dampers should be placed across the full width of the unit. High Performance of HVAC systems 55 Department of Thermal Engineering  2. Air Handling Units (AHU) MAIN COMPONENTS OF AN AHU 5. MIXING SECTION  An air-side economizer uses outdoor air to reduce refrigeration requirements: Advantage of cool outdoor air to assist mechanical cooling. Temperature control systems can modulate outdoor air and return air in the correct proportion without mechanical cooling. To exhaust the extra outdoor air a method of variable-volume relief must be provided: (i) modulate the relief air dampers in response to indoor space pressure or (ii) open relief/exhaust and air intake dampers simultaneously. A powered relief or return/relief fan may also be used, so that the relief system is off and relief dampers are closed when the air-side economizer is inactive. High Performance of HVAC systems 56 Department of Thermal Engineering  2. Air Handling Units (AHU) MAIN COMPONENTS OF AN AHU 5. MIXING SECTION  Static air mixers are designed to enhance mixing in the mixing plenum: Reduction or elimination of problems associated with stratification. Creation of turbulence in the airstream with no moving parts. Usually mounted between the mixing box and the heating or cooling coil. Static air mixer High Performance of HVAC systems 57 Department of Thermal Engineering  2. Air Handling Units (AHU) MAIN COMPONENTS OF AN AHU 6. OTHER SECTIONS AHU with energy recovery unit Besides the afore-described sections, others can be used if required: Silencers Combustion chambers Heat recovery systems High Performance of HVAC systems 58 Department of Thermal Engineering  2. Air Handling Units (AHU) MAIN COMPONENTS OF AN AHU 6. OTHER SECTIONS https://www.youtube.com/watch?v=l5uEGwDyHf8 AHU heat recovery wheel High Performance of HVAC systems 59 Department of Thermal Engineering  2. Air Handling Units (AHU) AIR FLOWS OF AN AHU Air used in AHU can be classified according to its origin: SUPPLY AIR: air supplied to the ducts systems in order to be drawn to the rooms to be conditioned. RETURN AIR: air extracted from the conditioned spaces. OUTSIDE AIR: new air taken from the outside. AHU can be classified depending on the air types they handle, as follows: 100% RECIRCULATING AIR 100% OUTSIDE AIR MIXED AIR High Performance of HVAC systems 60 Department of Thermal Engineering  2. Air Handling Units (AHU) AIR FLOWS OF AN AHU Air intake and outtake Filter and inner dampers Fans Recuperator Humidifier Parts of an AHU High Performance of HVAC systems 61 Department of Thermal Engineering  2. Air Handling Units (AHU) AIR FLOWS OF AN AHU 100% RECIRCULATING AIR Return and supply air are the same, that is, air is not renewed and no air is taken from the outside. 100% recirculating air AHU High Performance of HVAC systems 62 Department of Thermal Engineering  2. Air Handling Units (AHU) AIR FLOWS OF AN AHU 100% OUTSIDE AIR The return air is directly thrown out to the outside and supply air is entirely outside air. 100% outside air AHU: with heat recovery (left) and without heat recovery (right) High Performance of HVAC systems 63 Department of Thermal Engineering  2. Air Handling Units (AHU) AIR FLOWS OF AN AHU MIXED AIR Part of the air is recirculated and part is taken from the outside. AHU with mixed air: with heat recovery (left) and without heat recovery (right) High Performance of HVAC systems 64 Department of Thermal Engineering  2. Air Handling Units (AHU) EXERCISE 1. AHU components An industrial unit consists of two floors: the ground floor is dedicated to carpentry works and the upper floor is used to show furniture. The cooling system of the upper floor needs to condition air supplied to the room, since this air possesses a low humidity degree. Which components should the AHU include? High Performance of HVAC systems 65 Department of Thermal Engineering  2. Air Handling Units (AHU) EXERCISE 1. AHU components An industrial unit consists of two floors: the ground floor is dedicated to carpentry works and the upper floor is used to show furniture. The cooling system of the upper floor needs to condition air supplied to the room, since this air possesses a low humidity degree. Which components should the AHU include? One or two fans sections, depending on if there is only air forward or also backward. A cooling heat exchanger, since the installation is a cooling installation. In order to eliminate dust and sawdust particles air may sweep along due to the activity of the carpentry, a pre-filter and a filter should be used. Since the humidity of air is low, a humidifier must be included. If besides recirculating air, outside air is to be employed, an air mixing chamber is necessary. High Performance of HVAC systems 66 Department of Thermal Engineering  6. HVAC II: Air Conditioning Systems 1. Introduction to air conditioning systems 2. Air Handling Units (AHU) 3. Air distribution systems 4. Terminal units for air conditioning systems 5. Condensing loops 6. Control of air conditioning systems Table of contents 67  3. Air distribution systems Ductwork should deliver conditioned air to an area as directly, quietly and economically as possible. DESIGN CONSIDERATIONS  Ductwork sizing is often performed manually for simple systems, but commercially available duct- sizing software programs are often used for larger and complex systems.  Structural and architectural features of the building generally require some compromise and often limit the depth of the space available.  Duct systems can be designed for high or low velocity: A high-velocity system has smaller ducts: lower space requirements, higher pressures and more noise. In some low-velocity systems, medium or high fan pressures may be required to overcome high pressure drops. High Performance of HVAC systems 68 Department of Thermal Engineering  3. Air distribution systems DESIGN CONSIDERATIONS  In any variable-flow system, changing operating conditions can cause airflow to differ from the design flow.  In many applications the space between a suspended ceiling and the floor slab is used as a return air plenum. https://youtu.be/5y_VBiTiuAY High Performance of HVAC systems 69 Department of Thermal Engineering  3. Air distribution systems  All-air systems are classified in two categories: Single-duct systems contain the main coils in series and a common distribution system. Either capacity varying mechanism can be used. All-air HVAC system: Single-duct (left) High Performance of HVAC systems 70 Department of Thermal Engineering  3. Air distribution systems  All-air systems are classified in two categories: Dual-duct systems contain the main coils in parallel or in series-parallel with either a separate cold and warm distribution system. Generally vary the supply air temperature. All-air HVAC system Dual-duct High Performance of HVAC systems 71 Department of Thermal Engineering  3. Air distribution systems SINGLE-DUCT SYSTEMS CONSTANT VOLUME (CAV)  While maintaining constant airflow, these systems change the supply air temperature in response to the space load. Constant-volume system with reheat High Performance of HVAC systems 72 Department of Thermal Engineering  3. Air distribution systems SINGLE-DUCT SYSTEMS CONSTANT VOLUME (CAV)  Single-zone systems are the simplest all-air supply unit: Installed either in or remote from the space it serves. Short ductwork with low pressure drop, low fan energy. Possibility of shutting down without affecting adjacent areas: energy savings. A return or relief fan may be needed, depending on (i) system capacity and (ii) whether 100% outdoor air is All-air HVAC system for single zone used. High Performance of HVAC systems 73 Department of Thermal Engineering  3. Air distribution systems SINGLE-DUCT SYSTEMS CONSTANT VOLUME (CAV)  Multiple-zone reheat systems is a modification of the single-zone system. It provides zone or space control for areas of unequal loading. It provides simultaneous heating or cooling of perimeter areas with different exposures. Close control for process or comfort applications. Heat is added as a secondary simultaneous process to either preconditioned primary air or recirculated room air. Relatively small low-pressure system place reheat coils in the ductwork at each zone. High Performance of HVAC systems 74 Department of Thermal Engineering  3. Air distribution systems SINGLE-DUCT SYSTEMS CONSTANT VOLUME (CAV)  Multiple-zone reheat systems is a modification of the single-zone system. Conditioned air from a central unit is used, at a temperature low-enough to meet the maximum cooling load. Heat is added to the airstream in each zone to avoid overcooling. When a reheat system heats a space with exterior exposure the reheat coil must replace the heat lost from the space Single duct system with reheat terminal devices and bypass units. and offset cooling. High Performance of HVAC systems 75 Department of Thermal Engineering  3. Air distribution systems SINGLE-DUCT SYSTEMS VARIABLE AIR VOLUME (VAV)  A Variable Air Volume (VAV) system controls temperature in a spacy by varying the quantity of air supply: A VAV terminal unit at the zone varies the quantity of supply air to the space. The supply temperature is held relatively constant. Temperature must always be low enough to meet the cooling load in the most demanding zone. VAV systems can be applied to interior or perimeter zones with or without common fans, air temperature control and auxiliary heating devices. The greatest energy saving of a VAV occurs in perimeter zones. High Performance of HVAC systems 76 Department of Thermal Engineering  3. Air distribution systems SINGLE-DUCT SYSTEMS VARIABLE AIR VOLUME (VAV) Variable-Air-Volume system High Performance of HVAC systems 77 Department of Thermal Engineering  3. Air distribution systems SINGLE-DUCT SYSTEMS VARIABLE AIR VOLUME (VAV) Variable-Air-Volume system with reheat and induction and fan-powered devices High Performance of HVAC systems 78 Department of Thermal Engineering  3. Air distribution systems SINGLE-DUCT SYSTEMS VARIABLE AIR VOLUME (VAV)  A Variable Air Volume (VAV) system controls temperature in a spacy by varying the quantity of air supply: Humidity control in critical applications is a potential problem with VAV systems. Acceptable humidity levels can be achieved while maintaining minimum air circulation during reduced load by (i) raising the supply air temperature, (ii) providing auxiliary heat in each room independent of the air system, (iii) using individual-zone recirculation, (iv) recirculating air with a VAV induction unit, or (v) providing a dedicated recirculation fan to increase airflow. High Performance of HVAC systems 79 Department of Thermal Engineering  3. Air distribution systems SINGLE-DUCT SYSTEMS VARIABLE AIR VOLUME (VAV)  The Dual-Conduit system is an extension of the single-duct VAV: One supply duct offsets exterior transmission cooling or heating load by its terminal unit. The other supply air path provides cooling throughout the year. The primary air operates as a constant-volume system and the air temperature is varied to offset transmission only. The primary air fan is often limited to operating only during peak heating and cooling periods. The secondary air is cool year-round and varies in volume to match the load from solar heating and internal gains, and serves both perimeter and interior spaces. High Performance of HVAC systems 80 Department of Thermal Engineering  3. Air distribution systems SINGLE-DUCT SYSTEMS VARIABLE AIR VOLUME (VAV) Dual-Conduit system High Performance of HVAC systems 81 Department of Thermal Engineering  3. Air distribution systems SINGLE-DUCT SYSTEMS VARIABLE AIR VOLUME (VAV)  The discharge aperture of a Variable Diffuser is reduced to keep discharge velocity constant while reducing supply flow: The diffuser’s induction effect is kept high, cold air mixes in the space, and the room air distribution pattern is more nearly maintained at reduced loads. These devices are of two basic types: (i) one has a flexible bladder that expands to reduce the aperture and (ii) the other has a diffuser plate that moves. Both type of devices are pressure dependent and can be either powered by the system, electrically or pneumatically. High Performance of HVAC systems 82 Department of Thermal Engineering  3. Air distribution systems DUAL-DUCT SYSTEMS  A dual-duct system conditions all-air in a central apparatus and distributes it to conditioned spaces through two ducts: One ducts carries cold air an the other carries warm air. In each conditioned zone, air valve terminals mix warm and cold air in proper proportion to satisfy the space temperature and pressure control. Dual-duct systems may be designed as constant or variable volume: a dual-duct constant-volume system generally uses more energy. VAV systems can cause high relative humidity in the space during the cooling season. High Performance of HVAC systems 83 Department of Thermal Engineering  3. Air distribution systems DUAL-DUCT SYSTEMS CONSTANT VOLUME  Dual-duct, single-fan applications can be of two types: Single fan with reheat differ to a conventional reheat system in: (i) reheat is applied at a central point in the fan unit hot deck, and (ii) only part of the supply air is cooled; the rest is heated by the hot-deck coil. Single fan without reheat systems have no heating coil in the fan unit hot deck and simply pushes a mixture outdoor and recirculated air through the hot deck. High Performance of HVAC systems 84 Department of Thermal Engineering  3. Air distribution systems DUAL-DUCT SYSTEMS VARIABLE AIR VOLUME  Dual-duct VAV systems blend cold and warm air in various volume combinations.  These systems may include single-duct VAV terminal units connected to the cold-air duct distribution system for cooling-only interior spaces, and the cold duct may serve perimeter spaces in sync with the hot duct.  Modern dual-duct air terminals provide two damper operators per air terminal: the unit functions like a single-duct VAV cooling terminal unit and a single-duct VAV heating terminal unit in one physical terminal package.  This provides temperature control by means of a dual-duct box mixing at minimum airflow. High Performance of HVAC systems 85 Department of Thermal Engineering  3. Air distribution systems DUAL-DUCT SYSTEMS VARIABLE AIR VOLUME  Single-Fan, Dual-duct VAV systems uses a single-fan sized for the peak cooling load or the coincident peak of the hot and cold decks..  Fan control is from two static-pressure controllers: one located in the hot deck and the other in the cold deck.  The duct requiring the highest pressure governs the airflow by signalling the supply fan VFD speed control.  This provides temperature control by means of a dual-duct box mixing at minimum airflow.  The cold deck is normally maintained at a fixed temperature, although some central systems allow the temperature to rise during warmer weather to save refrigeration.  The hot deck temperature is often raised during periods of low outdoor temperature and high humidity to increase the flow over the cold deck for dehumidification. High Performance of HVAC systems 86 Department of Thermal Engineering  3. Air distribution systems DUAL-DUCT SYSTEMS VARIABLE AIR VOLUME Single-fan dual-duct system High Performance of HVAC systems 87 Department of Thermal Engineering  3. Air distribution systems DUAL-DUCT SYSTEMS VARIABLE AIR VOLUME  In Dual-Fan, Dual-duct VAV systems supply air volume of each supply fan in controlled independently.  The return fan in controlled based on the sum of the hot and cold fan volumes using flow-measuring stations.  Each fan is sized for the anticipated maximum coincident hot or cold volume, not the sum of the instantaneous peaks.  The cold deck can be maintained at a constant temperature either with mechanical refrigeration or with an economizer.  The heating coil need operate only when heating requirements cannot be met using return air. High Performance of HVAC systems 88 Department of Thermal Engineering  3. Air distribution systems DUAL-DUCT SYSTEMS VARIABLE AIR VOLUME Dual-fan, dual-duct system High Performance of HVAC systems 89 Department of Thermal Engineering  3. Air distribution systems MULTIZONE SYSTEMS All-air HVAC system for multiple zones High Performance of HVAC systems 90 Department of Thermal Engineering  3. Air distribution systems MULTIZONE SYSTEMS  The multizone system supplies several zones from a single, centrally located AHU.  Different zone requirements are met by mixing cold and warm air through zone dampers in response to thermostats.  The multizone system is similar to the dual-duct system and has the same potential problem with high humidity.  Packaged equipment is usually limited to about 12 zones.  A multizone system is more energy efficient than a terminal reheat because not all the air goes through the cooling coil.  The multizone system uses essentially the same fan energy as terminal reheat because the airflow is constant. High Performance of HVAC systems 91 Department of Thermal Engineering  3. Air distribution systems MULTIZONE SYSTEMS Multizone system High Performance of HVAC systems 92 Department of Thermal Engineering  3. Air distribution systems MULTIZONE SYSTEMS  In a three-deck multizone system bypass zone dampers are installed in the AHU parallel with the hot and cold deck dampers.  Hot and cold air are never mixed in the three-deck system.  Zones that require cooling receive a mixture of cold and neutral air, and zones that require heating receive a mixture of hot and neutral air.  The neutral air will take the place of the hot-deck air, eliminating the need for the heating coil in summer.  In winter, the neutral air will be cooler than the hot deck, thus replacing the cold deck and the need for activating the cooling coil. High Performance of HVAC systems 93 Department of Thermal Engineering  3. Air distribution systems MULTIZONE SYSTEMS Three-deck multizone system High Performance of HVAC systems 94 Department of Thermal Engineering  6. HVAC II: Air Conditioning Systems 1. Introduction to air conditioning systems 2. Air Handling Units (AHU) 3. Air distribution systems 4. Terminal units for air conditioning systems 5. Condensing loops 6. Control of air conditioning systems Table of contents 95  4. Terminal units for air conditioning systems  Air terminal units (ATUs) are located between the primary air distribution systems and the conditioned space.  There are two main types: Passive devices, deliver and extract air throughout the conditioned space without occupants sensing a draft and without creating excessive noise. In active devices, also known as boxes, the ATU controls the quantity and/or temperature of the supply air to maintain desired conditions. High Performance of HVAC systems 96 Department of Thermal Engineering  4. Terminal units for air conditioning systems  In low velocity all-air systems air may enter from the supply air ductwork directly into the conditioned space through a grille or diffuser. Air-flow amounts and directions are usually manually adjusted.  In medium and high velocity air systems and intermediate device controls air volume, reduces duct pressure, or both: Constant volume terminal unit. Variable Air Volume (VAV) terminal unit, which varies the amount of air delivered to the space. All-air induction terminal unit, which controls the volume of primary air. Air-water induction terminal unit, which includes a coil in the induced airstream. Fan-powered terminal unit, which uses an integral fan to accomplish the mixing. https://youtu.be/MqM-U8bftCI High Performance of HVAC systems 97 Department of Thermal Engineering  4. Terminal units for air conditioning systems CONSTANT VOLUME ATU WITH REHEAT https://youtu.be/XgQ3v6lvoZQ  Constant-volume reheat ATUs are mainly used in terminal reheat systems.  The unit serves as pressure-reducing valve and constant- volume regulator. High Performance of HVAC systems 98 Department of Thermal Engineering  4. Terminal units for air conditioning systems CONSTANT VOLUME ATU WITH REHEAT  It is generally fitted with an integral reheat coil for space temperature control: The constant supply air quantity is selected to provide cooling to match the peak load. The reheat coil is controlled to raise the supply temperature at reduced loads. Single-duct terminal unit with reheat High Performance of HVAC systems 99 Department of Thermal Engineering  4. Terminal units for air conditioning systems VARIABLE AIR VOLUME  VAV terminal units control the space temperature by varying the volume of air supply from the AHU and are available in many configurations.  These units are fitted with automatic controls that are either pressure dependent or pressure independent: Pressure-dependent units control damper position in response to room temperature, and flow may increase and decrease as the main duct pressure varies. Pressure-independent units measure actual supply airflow and control flow in response to room temperature. A velocity-limit control may included to prevent excess airflow. High Performance of HVAC systems 100 Department of Thermal Engineering  4. Terminal units for air conditioning systems VARIABLE AIR VOLUME THROTTLING WITHOUT REHEAT  Throttling without reheat essentially uses an air valve or damper to reduce supply airflow to the space in response to falling loads.  The unit usually includes some means of sound attenuation to reduce air noise created in the throttling action.  It is the simplest and least expensive VAV terminal unit. High Performance of HVAC systems 101 Department of Thermal Engineering  4. Terminal units for air conditioning systems VARIABLE AIR VOLUME THROTTLING WITH REHEAT  This system integrates heating at the terminal unit (electric or hot water) with the same type of air valve.  It is applied in interior and exterior zones where full heating and cooling flexibility is required.  VAV with reheat allows airflow to be reduced as the first step in control; heat is then initiated as the second step Variable-Air-Volume box with reheat High Performance of HVAC systems 102 Department of Thermal Engineering  4. Terminal units for air conditioning systems VARIABLE AIR VOLUME DUAL-DUCT  Dual-duct systems typically feature throttling dual-duct VAV air terminal units.  These ATUs are very similar to the single-duct VAV except that two primary air inlets are provided.  Minimum positions are available on these dampers to meet minimum ventilation airflow requirements. Dual-duct VAV terminal unit High Performance of HVAC systems 103 Department of Thermal Engineering  4. Terminal units for air conditioning systems VARIABLE AIR VOLUME INDUCTION  The VAV induction system uses an ATU to reduce cooling capacity by simultaneously reducing primary air and inducing room or ceiling air to maintain a relatively constant supply temperature.  The primary-air quantity decreases with load, retaining savings of VAV, and the air supplied to the space is kept relatively constant.  VAV induction units require a higher inlet static pressure. Section through an induction diffuser unit  Induction units have been displaced by fan-powered ATUs. High Performance of HVAC systems 104 Department of Thermal Engineering  4. Terminal units for air conditioning systems VARIABLE AIR VOLUME INDUCTION High Performance of HVAC systems 105 Department of Thermal Engineering  4. Terminal units for air conditioning systems VARIABLE AIR VOLUME FAN-POWERED  Fan-powered systems can have either parallel or series airflow: In parallel-flow units the fan is located outside the primary airstream to allow intermittent fan operation. A backdraft damper on the terminal fan prevents conditioned air from escaping into the return air plenum when the fan is off. In series units the fan is located in the primary airstream and runs continuously when the zone is occupied. They can help maintain indoor air quality by recirculating air from over-ventilated zones to zones with greater requirements. High Performance of HVAC systems 106 Department of Thermal Engineering  4. Terminal units for air conditioning systems VARIABLE AIR VOLUME FAN-POWERED https://youtu.be/vw-bAbjPTd8  Fans for fan-powered AHUs operated in series are sized and operated to maintain minimum static pressures at the unit inlet connections  more fan energy than a throttling unit system.  They may generate higher sound levels than throttling ATUs.  A disadvantage of this type of terminal unit is the need to periodically change filters  unsuitable for installation above inaccessible ceilings.  Fan-powered units can avoid using reheat coils in internal spaces since they are usually located in the ceiling plenum to recover heat from lights. VAV box with terminal reheat High Performance of HVAC systems 107 Department of Thermal Engineering  4. Terminal units for air conditioning systems GRILLES AND DIFFUSERS  Conditioned air is propelled to the spaces through grilles or diffusers.  Grilles provide air in a single plane and direction, according to how louvers are oriented.  Grilles achieve great air impulsion distances, but can cause discomfort due to the generation of a direct air flow over a point or zone.  Diffusers supply air in various planes and directions, and the space temperature is more homogeneous.  Diffusers do not cause discomfort on occupants, but the impulsion distance is more reduced.  When selecting grilles and diffusers the pressure drops generated must be taken into consideration. High Performance of HVAC systems 108 Department of Thermal Engineering  4. Terminal units for air conditioning systems GRILLES AND DIFFUSERS  Grilles are usually made of aluminium, steel or plastic materials, and are installed in walls and celings.  Grilles are typically of the following types: Single / Double deflection Fix / Adjustable louvers Single (left) and double (right) deflection grilles Grilles with fix (left) and adjustable (right) louvers High Performance of HVAC systems 109 Department of Thermal Engineering  4. Terminal units for air conditioning systems GRILLES AND DIFFUSERS  Diffusers are usually made of aluminium or steel, and the following are the most common: Ceiling diffusers (circular and square) Linear slot diffusers for ceilings or walls Rotational diffusers Circular ceiling diffuser (left) and linear slot diffuser (right) https://youtu.be/zephL3PidMI Rotational flow diffusers High Performance of HVAC systems 110 Department of Thermal Engineering  6. HVAC II: Air Conditioning Systems 1. Introduction to air conditioning systems 2. Air Handling Units (AHU) 3. Air distribution systems 4. Terminal units for air conditioning systems 5. Condensing loops 6. Control of air conditioning systems Table of contents 111  5. Condensing loops How can we exchange heat in buildings with a high volume/roof relationship? In this case there is no room for all the necessary chillers. We need to use condensing loops. High Performance of HVAC systems 112 Department of Thermal Engineering cooling-towers.webp 5. Condensing loops High Performance of HVAC systems 113 Department of Thermal Engineering  5. Condensing loops High Performance of HVAC systems 114 Department of Thermal Engineering  5. Condensing loops  The compression heat of the vapour-compression cycle must be rejected. Air-cooled systems. Water cooled systems.  Condenser water systems can be of two types: Recirculating or cooling tower systems. Once-through systems: city water, well-water, lake/groundwater systems.  Water consumption rate of a cooling tower is about 5% of the once-through system.  The amount of heated water discharged is very small  ecological effect reduced.  Cooling towers can cool water 20 K lower than air-cooled systems aprox.  increase of the efficiency of the overall system. https://youtu.be/UmWWZdJR1hQ High Performance of HVAC systems 115 Department of Thermal Engineering  5. Condensing loops  A cooling tower cools water by a combination of heat and mass transfer. Water to be cooled is distributed in the tower by spray nozzles, splash bars, or film-type fill, exposing a very large water surface are to atmospheric air. Atmospheric air is circulated by fans, convective currents, natural wind currents, or induction effect from sprays. A portion of water absorbs heat to change from liquid to vapour state. The heat of vaporization at atmospheric pressure is transferred from the liquid water into the airstream.  The range (TA-TB) is determined by the heat load and water flow rate.  The approach is a function of the cooling tower capability. Temperature relationship between water and air in conterflow cooling tower High Performance of HVAC systems 116 Department of Thermal Engineering  5. Condensing loops  Thermal performance of a cooling tower depends mainly on the entering air wet-bulb temperature.  The amount of heat transferred from the water to the air is proportional to hB-hA: Latent heat exchange: hC-hA. Sensible heat exchange: hB-hC.  If the entering air dry-bulb temperature is changed to point D at the same wet-bulb temperature, the total heat transfer remains the same.  Mass transfer (evaporation) is proportional to the change in specific humidity. Psychrometric analysis of air  The evaporation rate at typical design conditions is approximately of 1% of passing through cooling tower the water flow rate for each 7 K of water temperature range. High Performance of HVAC systems 117 Department of Thermal Engineering  5. Condensing loops  Two basic types of evaporative cooling devices are used: Direct-contact or open cooling tower: exposes water directly to the cooling atmosphere. Closed-circuit cooling tower: involves indirect contact between heated fluid and atmosphere. Direct-contact (left) and indirect-contact (right) evaporative cooling towers. High Performance of HVAC systems 118 Department of Thermal Engineering  6. HVAC II: Air Conditioning Systems 1. Introduction to air conditioning systems 2. Air Handling Units (AHU) 3. Air distribution systems 4. Terminal units for air conditioning systems 5. Condensing loops 6. Control of air conditioning systems Table of contents 119  6. Control of air conditioning systems AIR DISTRIBUTION SYSTEMS CONTROLS  Controls should be automatic and simple for best operating and maintenance efficiency.  Depending on the space need, one controlling thermostat closes a normally open heating valve, opens the outdoor air mixing dampers, or opens the cooling valve.  Air handling systems should include means to measure and control the amount of outdoor air being brought: Separate contant-volume 100% outdoor Modulating the return damper to maintain a air ventilation systems constant pressure drop across a fixed outdoor orifice Outdoor air injection fan Airflow-measuring systems that maintain a constant difference between supply and return Directly measuring the outdoor air flow CO2 and/or VOC-based demand-controlled rate ventilation High Performance of HVAC systems 120 Department of Thermal Engineering  6. Control of air conditioning systems AIR DISTRIBUTION SYSTEMS CONTROLS Minimum outdoor air control with (left) outdoor air injection fan and (right) differential pressure controls High Performance of HVAC systems 121 Department of Thermal Engineering  6. Control of air conditioning systems AIR DISTRIBUTION SYSTEMS CONTROLS Minimum outdoor air control with measuring stations High Performance of HVAC systems 122 Department of Thermal Engineering  6. Control of air conditioning systems AIR DISTRIBUTION SYSTEMS CONTROLS  Proper selection of outdoor, relief, and return air dampers is critical for efficient operation  oversized dampers are unable to control.  A mixed-air temperature control can reduce operating costs and temperature swings from load variations.  Direct digital control (DDC) is common, and most manufacturers offer DDC package for equipment.  DDC controls possess the ability for recording actual energy consumption or other operating parameters that can be helpful for optimizing control strategies. High Performance of HVAC systems 123 Department of Thermal Engineering  6. Control of air conditioning systems AIR DISTRIBUTION SYSTEMS CONTROLS CONSTANT-VOLUME REHEAT  This system typically uses two subsystems for control: One controls the discharge air conditions from the AHU. The other maintains the space conditions by controlling the reheat coil. High Performance of HVAC systems 124 Department of Thermal Engineering  6. Control of air conditioning systems AIR DISTRIBUTION SYSTEMS CONTROLS VARIABLE AIR VOLUME  Air volume can be controlled by (i) duct-mounted terminal units serving multiple air outlets in a control zone or (ii) by units integral to each supply air outlet.  Pressure-independent volume-regulator units control flow in response to the thermostat’s call for heating or cooling. The required flow is maintained regardless of fluctuation of the system pressure. Adjustable for maximum and minimum (or shutoff) air settings.  Pressure-dependent devices control air volume in response to a unit thermostatic device, but flow varies with the inlet pressure variation. Less expensive High Performance of HVAC systems 125 Department of Thermal Engineering  6. Control of air conditioning systems AIR DISTRIBUTION SYSTEMS CONTROLS VARIABLE AIR VOLUME Single-zone VAV system and control High Performance of HVAC systems 126 Department of Thermal Engineering  6. Control of air conditioning systems AIR DISTRIBUTION SYSTEMS CONTROLS VARIABLE AIR VOLUME  The type of controls available for VAV units varies with the terminal device: most use either pneumatic or electric controls and may be either self-powered or system-air actuated: Self-powered controls position the regulator by using liquid or wax-filled power elements System-powered devices use air from the air supplied to the space to power the operator  Fan control for power conserving and noise limitation: Fan speed control Fan discharge damper Variable inlet vane control Variable-pitch fan control Fan bypass High Performance of HVAC systems 127 Department of Thermal Engineering  6. Control of air conditioning systems AIR DISTRIBUTION SYSTEMS CONTROLS VARIABLE AIR VOLUME Induction VAV terminal unit (left), series fan-power VAV terminal unit (middle) and parallel fan terminal unit (right) High Performance of HVAC systems 128 Department of Thermal Engineering  6. Control of air conditioning systems AIR DISTRIBUTION SYSTEMS CONTROLS DUAL DUCT  Dual-duct systems are generally more costly to install  less widespread use DDC control systems can make dual-duct systems worthwhile for certain applications System-powered devices use air from the air supplied to the space to power the operator Variable-volumen dual-duct terminal unit High Performance of HVAC systems 129 Department of Thermal Engineering  6. Control of air conditioning systems AIR DISTRIBUTION SYSTEMS CONTROLS AIR-SIDE ECONOMIZER CYCLE  Relief or exhaust fans can be started in response to a signal from the economizer control or to a space pressure controller: the main supply fan must be able to handle the return air pressure drop.  High-limit controls are intented to disable the economizer: Fixed dry-bulb temperature Differential dry-bulb temperature Fixed enthalpy Integrated economizer cycle control Differential enthalpy High Performance of HVAC systems 130 Department of Thermal Engineering  6. Control of air conditioning systems AUTOMATIC CONTROLS AND BUILDING MANAGEMENT SYSTEMS  Central AHUs increasingly come with prepackaged and prewired automatic control systems. DDC control systems can make dual-duct systems worthwhile for certain applications System-powered devices use air from the air supplied to the space to power the operator  The next level of HVAC system management is to integrate manufacturers’ control packages with the building management system (BMS).  Automatic temperature controls can be important in establishing a simple or complex control system.  The BMS can be an important business tool in achieving sustainable facility management success. High Performance of HVAC systems 131 Department of Thermal Engineering  6. Air conditioning systems ASHRAE “ ASHRAE Handbook - HVAC Systems and Equipment”, 2016. ASHRAE “ ASHRAE Handbook – HVAC Applications”, 2015. ASHRAE Learning Institute, “Fundamentals of HVAC”, 2006. ATECYR, “Fundamentos de Climatización”, 2010. Antonio Jesús Mendoza Ramírez, “Eficiencia energética en las instalaciones de climatización en los edificos, 2013. Shaimaa Seyam “Types of HVAC Systems”, 2018. References 132 An Erasmus Mundus International Joint Master Degree