HVAC Fundamentals Quiz

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.

Varying the temperature of the supply air (Constant Air Volume - CAV) and varying the amount of the supply air (Variable Air Volume - VAV).

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.

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.

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.

Lower humidity levels, Reduced fan energy consumption.

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…

Major equipment in an unoccupied area, Noise-producing equipment away from the occupied area, Great potential to use outdoor air for economizer cooling, Simple seasonal changeover, Wide choice of zoning, flexibility, and humidity control, Heat recovery may be readily incorporated, 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

Ducts installed in ceiling plenums require additional clearance, Larger floor plans to allow adequate space for vertical shafts, Transport energy larger than in other systems, Equipment rooms represent nonrentable spaces, More difficult accessibility to terminal devices, dampers, etc., Failure of a central component affects all zones served.

Exterior zones are affected by weather conditions and may require both heating and cooling at different times.

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.

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.

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.

Air Handling Units (AHU) are used in big installations to maintain air conditions (temperature, humidity, air quality) within desired ranges.

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 cooling generators.

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.

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, Outdoor air requirements, Desired space pressures, Heating and cooling coil capacities, Required pressure capabilities of the fan(s).

The type of facility and other factors will determine where the air-handling equipment is located.

One important design decision: central or decentralized MER?

Fewer total pieces of equipment to maintain, Maintenance is concentrated at one location, Filtration is easily enhanced, Energy recovery opportunities may be more practical, Vibration and noise control and equipment more simple to handle.

Reduced size of ducts: less space required for ductwork, Reduced equipment size: cheaper and less sophistication, Possibility of turning off in unoccupied areas, Failure of an AHU affects only part of the building.

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.

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.

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.

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.

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.

This section is responsible for propelling air to be conditioned through the different sections of the AHU.

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.

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.

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.

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.

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.

The efficiency of a fan is the ratio between the power transferred to the air flow and the power used by the fan: η = 𝜌 · 𝑉 · 𝑐 · ∆𝑇 / 𝑊. In certain applications, it may be desirable to calculate the temperature rise across the fan: ∆𝑇 = ∆𝑃 / 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.

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).

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.

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.

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.

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.

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.

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.

An air mixing chamber (box) is installed with two purposes: 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).

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.

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.

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.

Besides the afore-described sections, others can be used if required: Silencers, Combustion chambers, Heat recovery systems.

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.

Return and supply air are the same, that is, air is not renewed and no air is taken from the outside.

The return air is directly thrown out to the outside and supply air is entirely outside air.

Part of the air is recirculated and part is taken from the outside.