Topic 4.1 - Principles of Environmental Control (Psychometry and Air Conditioning) PDF

Topic Principles of Environmental Control Part - 1  Copyright Disclaimer Under section 185 of RA 8293 (Intellectual Property Code of the Philippines), allowance is made for “fair use” for purposes such as criticism, comment, news reporting, t...

Topic Principles of Environmental Control Part - 1  Copyright Disclaimer Under section 185 of RA 8293 (Intellectual Property Code of the Philippines), allowance is made for “fair use” for purposes such as criticism, comment, news reporting, teaching, scholarship, education and research. Fair use is a use permitted by copyright statute that might otherwise be infringing. This presentation has been adapted mainly from several references and literature published by local and international research institutions. For educational purposes only. No Copyright Infringement Intended.  Contents Psychrometry and Air Conditioning Insulation and Vapor Barriers Energy Considerations Psychometry and Air Conditioning  Psychometry and Air Conditioning INTRODUCTION ✓ The atmosphere in and around our homes and agricultural buildings primarily contains air and water vapor. ✓ High moisture levels, particularly when combined with high temperatures, seriously affect the comfort of both men and animals and the production of livestock as well. ✓ Controlling the moisture level in the atmosphere is one of the more important aspects of maintaining a desirable environment in farm buildings.  Psychometry and Air Conditioning PSYCHROMETRY ✓ This branch of science deals with the study of properties of moist air and its behavior under different conditions. ✓ The properties of moist air include Dry-bulb Temperature (DBT), Wet-bulb Temperature (WBT), humidity, Relative Humidity (RH), etc.  Psychometry and Air Conditioning PSYCHROMETRY Dry-Bulb Temperature (DBT) It is the temperature of air measured or recorded by a thermometer. It is denoted by T or Tdb or DBT. Wet-Bulb Temperature (WBT) It is the temperature of air recorded by a thermometer when its bulb is covered with wet wick or cloth over which air is moving at a velocity at 2.5 to 10 m/s. Dew Point Temperature (DPT) It is the temperature of air recorded by a thermometer when the moisture present in it starts condensing. It is denoted by Tdp or DPT.  Psychometry and Air Conditioning PSYCHROMETRY Dew Point Temperature (DPT) In the case of dehumidification of air, it is required to maintain the temperature of cooling coil well below DPT. During the cooling process, the partial pressure of water vapor and the specific humidity w remain constant until the vapor starts condensing. The DPT (saturated temperature) can be found from the steam table corresponding to the partial pressure of water vapor Pv.  Psychometry and Air Conditioning FUNDAMENTAL PARAMETERS ❖ Pressure ❖ Specific Humidity or Humidity Ratio ❖ Relative Humidity ❖ Enthalpy ❖ Specific Volume ❖ Degree of Saturation  Psychometry and Air Conditioning FUNDAMENTAL PARAMETERS ❖ Pressure ✓ In air conditioning terms, air means a mixture of water vapor and remaining gases. ✓ So, by Dalton's law of partial pressure where, p = total pressure of air pa = partial pressure of dry air pv = partial pressure of water vapor.  Psychometry and Air Conditioning FUNDAMENTAL PARAMETERS ❖ Pressure ✓ Carrier's equation can find out the partial pressure of water vapor where pw = saturation pressure of water vapor corresponding to wet bulb temperature (from steam tables) pv = atmospheric pressure of moist air Tdb = dry bulb temperature Twb = wet bulb temperature.  Psychometry and Air Conditioning FUNDAMENTAL PARAMETERS ❖ Specific Humidity or Humidity Ratio ✓ It is the ratio of the mass of water vapor to the mass of dry air contained in the sample air. It is denoted by w. It is typically expressed in kg/kg of dry air. w = mass of water vapor in air/mass of dry air in air = mv/ma Let pa, va T, ma, and Ra be the pressure, specific volume, DBT in K, mass, and gas constant, respectively. Let pv, vv T, mv, and Rv be the pressure, specific volume, DBT in K, mass, and gas constant, respectively.  Psychometry and Air Conditioning ❖ Specific Humidity or Humidity Ratio As air and water vapor have the same volume and temperature, we get  Psychometry and Air Conditioning ❖ Specific Humidity or Humidity Ratio where pv and pa are partial pressure of water vapor and dry air respectively.  Psychometry and Air Conditioning ❖ Specific Humidity or Humidity Ratio Example: The dry-bulb and dew point temperatures of atmospheric air are 30°C and 14°C, respectively. Determine the humidity ratio if the barometer reading is 758 mm of Hg.  Psychometry and Air Conditioning ❖ Specific Humidity or Humidity Ratio Note: Use of steam table Solution: From steam tables, the partial pressure of water vapor corresponding to DP temperature 14°C is 0.015973 bar. Atmospheric pressure is 758 mm Hg (758 x 0.001332 = 1.0105656 bar).  Psychometry and Air Conditioning ❖ Relative Humidity ✓ It is the ratio of the mass of water vapor in a given air volume at any temperature and pressure to the maximum amount of water vapor that the same volume of air can hold at the same temperature conditions. ✓ The air contains the maximum amount of water vapor at the saturation conditions  Psychometry and Air Conditioning ❖ Relative Humidity ✓ Relative humidity is therefore defined as the ratio of vapor pressure in a sample of air to vapor pressure of saturated air at the same temperature, i.e., RH = vapor pressure of water vapor/vapor pressure of saturated air at the same temperature ✓ Relative humidity is measured in percentage. It has a significant influence on the evaporation of water in the air and therefore on the comfort of human beings.  Psychometry and Air Conditioning ❖ Enthalpy ✓ Air is a homogeneous mixture of dry air and water vapor. Therefore, the enthalpy of air is found by taking the sum of dry air's enthalpy and water vapor's enthalpy in the moist air. ✓ Enthalpy of air/kg of dry air = Enthalpy of dry air + enthalpy of w kg of water vapor = ha + whv ✓ Considering the change in enthalpy of perfect gas as a function of temperature only, the enthalpy of dry air part, above a datum of 0°C, can be found as ha = cpaTdb = 1.005 Tdb kJ/kg  Psychometry and Air Conditioning ❖ Specific Volume ✓ It is the volume of air per unit mass of dry air. It is measured in m³/kg of dry air. ✓ Air flow is measured by an anemometer as the volume rate of flow and the heat added or cooling requires mass flow rate. ✓ So, specific volume is essential to relate the two.  Psychometry and Air Conditioning ❖ Degree of Saturation ✓ It is the mass of water vapor in a sample of air to the mass of water vapor in the same air when it is saturated at the same temperature. Mathematically, Where wv and ws are specific humidity of air and saturated air, respectively where, ps = partial pressure of water vapor when air is separated. It is obtained from steam tables corresponding to DBT (Tdb ) pv = partial pressure of water vapor in moist air p = total pressure of moist air  Psychometry and Air Conditioning ❖ Degree of Saturation ✓ Relative humidity, RH = pv/ps = 0 when moist air is totally dry, i.e. which does not contain water vapor. ✓ If the moist air is saturated, then pv=ps then RH = 1 and µ = 1. It shows that the degree of saturation varies between 0 and 1.  Psychometry and Air Conditioning ❖ PSYCHROMETRIC CHART ✓ A psychrometric chart is a graphical representation of the thermodynamic properties of moist air. ✓ These properties of moist air vary with atmospheric pressure and altitude. ✓ One such chart for atmospheric pressure of 1.01325 bar at sea level is shown in the following figure.  Psychometry and Air Conditioning ❖ PSYCHROMETRIC CHART ✓ The variables shown on a complete psychrometric chart are DBT, WBT, relative humidity, total heat, vapor pressure, and the actual moisture content of the air. ✓ As shown in this figure, the dry- bulb temperature is taken as the x- axis, and the mass of water vapor per kg of dry air as the ordinate.  Psychometry and Air Conditioning ❖ PSYCHROMETRIC CHART DBT lines These dry-bulb temperature lines extend vertically upwards and there is one line for each degree of temperature. WBT lines The wet-bulb temperature scale is found along the 'in-step' of the chart extending from the toe to the top. These lines extend diagonally downwards to the right. There is one line for each degree of temperature.  Psychometry and Air Conditioning ❖ PSYCHROMETRIC CHART RH lines ✓ On the psychrometric chart, the relative humidity lines are the only curved lines on it. ✓ The 100% RH line or saturation curve becomes the boundary of the chart on the left side. ✓ The region beyond this line is the supersaturated zone or fog zone.  Psychometry and Air Conditioning ❖ PSYCHROMETRIC CHART Specific humidity lines ✓ The scale for specific humidity is vertical on the right side of the psychrometric chart. ✓ The scale is in grams of moisture per kilogram dry air.  Psychometry and Air Conditioning ❖ PSYCHROMETRIC CHART DPT lines ✓ The scale for dew point temperature is identical to the scale of WBT lines. The DPT lines run horizontal to the right.  Psychometry and Air Conditioning ❖ PSYCHROMETRIC CHART Specific volume lines ✓ The specific volume lines are drawn along the sole chart and are equally- spaced diagonal lines.  Psychometry and Air Conditioning ❖ PSYCHROMETRIC CHART Specific enthalpy lines ✓ The specific enthalpy scale is located along the 'in-step' of the chart. These lines are similar to WBT lines ✓ Specific enthalpy lines indicate the total heat content.  Psychometry and Air Conditioning PSYCHROMETRIC PROCESSES OR AIR CONDITIONING PROCESSES ❖ Sensible Heating ❖ Sensible Cooling ❖ Cooling and Dehumidification ❖ Heating and Humidification  Psychometry and Air Conditioning Sensible Heating of Air ✓ Sensible heat will be added to the moist air while passing it over the hot, dry surface. ✓ Normally, the heating surface is a steam or hot water coil whose surface temperature is above the DBT of the air. ✓ As the air meets the warm surface, the DBT of air increases and tends to approach the temperature of the heating surface.  Psychometry and Air Conditioning Sensible Heating ✓ Since no moisture is added or removed from the air the specific humidity remains constant. ✓ The process is illustrated with a schematic diagram as in the figure. ✓ The process takes place along the constant DPT line. ✓ For steady flow conditions, the required rate of heat addition is  Psychometry and Air Conditioning Sensible Cooling of Moist Air ✓ Sensible cooling of moist air can be done by passing it over a cooling coil whose surface temperature is kept below the DBT of entering air and above the DP temperature. ✓ In this process, no moisture is added or removed, and DP temperature and latent heat content of air remain the same throughout the cooling process.  Psychometry and Air Conditioning Sensible Cooling of Moist Air ✓ The process is represented as a horizontal line from right to left, depending upon the end conditions of air (see figure). ✓ The total change in heat (enthalpy) content is equal to the change in sensible heat. ✓ Following figure shows the cooling process on psychrometric chart. ✓ The air can be cooled to surface temperature of coil. But this requires contact of air with coil surface for a sufficient period. ✓ The cooling of air depends on the number of rows of coil, depth of the coil and the velocity of air approaching the coil.  Psychometry and Air Conditioning Sensible Cooling of Moist Air ✓ The heat transfer between the air and the cooling coil indicates that the temperature difference between the air and the coil surface is large at the beginning (first few rows of the coil). At subsequent rows this difference decreases. So; the last few rows are uneconomical due to very small temperature drop achieved. ✓ Therefore, the number of rows is limited, and the air is then cooled to temperature higher than T2.  Psychometry and Air Conditioning Sensible Cooling of Moist Air ✓ Low velocity of air allows adequate time for cooling. ✓ But low velocity causes laminar flow and also requires a higher cross-section of coil for adequate airflow. ✓ So, velocity must be of reasonably high value. Thus, air cannot be cooled to coil temperature by low air velocity. Cooling capacity of coil = mass flow rate of air x specific heat x (T1 – T2)  Psychometry and Air Conditioning Cooling and Dehumidification of Moist Air ✓ Here air is to be cooled and during cooling, water vapor is to be separated from the air. ✓ Moisture separation will occur only when moist air is cooled to a temperature below its dew point temperature. ✓ Therefore, the effective surface temperature of the cooling coil kept below the initial dew point temperature of the air is called Apparatus Dew Point (ADP).  Psychometry and Air Conditioning Cooling and Dehumidification of Moist Air Although moisture separation occurs at various temperatures ranging from the initial dew point to final saturation temperature, it is assumed that the condensate water is cooled to the final air temperature T2 before it drains from the system under steady state. a) Device for cooling and dehumidification b) cooling & dehumidification on chart  Psychometry and Air Conditioning Cooling and Dehumidification of Moist Air Cooling the air to the coil surface temperature becomes uneconomical as the size of the heat exchanger becomes very large as (ΔT) reduces. Therefore, surface area of the coil is so designed that it will have a certain bypass factor given by following formula, ✓ In any cooling and dehumidification process, both sensible and latent heats need to be rejected and this is carried out by the cooling fluid circulated through the coil. ✓ The sum of sensible and latent heat is the total heat transferred. ✓ The ratio of sensible heat to the total heat transfer is known as the Coil Sensible Heat Factor (CSHF).  Psychometry and Air Conditioning Cooling and Dehumidification of Moist Air By knowing CSHF, we can find the sensible heat and latent heat quantities. The enthalpy difference (h1 – h2) represents the total heat absorbed by the coil. In the processes 1-2, 1-2', 1-2" the total heat absorbed is same but the proportion of sensible heat absorbed to total heat absorbed successively reduces. This is indicated by the fact that T1" > T2' > T2. This proportion is called the Sensible Heat Factor (SHF).  Psychometry and Air Conditioning Cooling and Dehumidification of Moist Air SHF represents the slope of the line representing the process on the psychrometric chart. So, from figure it can be said that,  Psychometry and Air Conditioning ADIABATIC COOLING OR COOLING WITH HUMIDIFICATION PROCESS ✓ Adiabatic cooling is because of the water attaining the wet-bulb temperature of air due to constant evaporation. ✓ In some dry areas, the difference between dry-bulb and wet-bulb temperature is high enough to use well water as cooling medium in a coil. ✓ As the WB temperature is higher than the dew point of air, the process is sensible cooling.  Psychometry and Air Conditioning ADIABATIC COOLING OR COOLING WITH HUMIDIFICATION PROCESS ✓ Let us consider warm air blowing over the water surface as shown in following figure. a) Adiabatic or evaporative cooling b) Schematic diagram  Psychometry and Air Conditioning ADIABATIC COOLING OR COOLING WITH HUMIDIFICATION PROCESS ✓ The air gets cooled. For cooling, air has to lose the heat it possesses. The air physically contacts the water surface, therefore, in one way it dissipates heat to the water by conduction. ✓ Secondly, water particles separate from the water surface, evaporate and mix with the air. For the evaporation of water latent heat is to be supplied which actually comes from both air as well as water. Therefore, both air and water get cooled. ✓ The heat exchange process is only between the air and the water, so the system is said to be insulated. This process of cooling is known as adiabatic cooling. It is also called evaporative cooling or cooling with humidification. ✓ For this reason, water in an open lake or well is cooler than the surrounding.  Psychometry and Air Conditioning ADIABATIC COOLING OR COOLING WITH HUMIDIFICATION PROCESS ✓ The water surface can also be in the form of water spray as shown in Figure (a) to hasten evaporation and cooling. ✓ During an adiabatic process no heat enters or leaves the system. Thus, the process line 1-2 on the psychrometric chart is along the constant enthalpy line. ✓ The lowest possible temperature to which air can be cooled is the wet-bulb temperature (Twb). ✓ Due to inefficient spray systems or uneconomical situations of providing a large number of banks of spray to get the cooling to Twb, air is practically cooled to T2 (T2 > Twb). ✓ The efficiency of spray is defined as  Psychometry and Air Conditioning ADIABATIC COOLING OR COOLING WITH HUMIDIFICATION PROCESS ✓ The water is recirculated with a pump. Make-up water is added to compensate for the water evaporated during the operation. ✓ The makeup water also known as humidifier duty in liter/h can be given by the formula,  Psychometry and Air Conditioning Heating and Humidification ✓ In this case, heat and water vapor are added to the air. ✓ To achieve this, the water to be sprayed in the air stream is kept at a temperature greater than the DBT of incoming air so that heat will be transferred to air to heat it. ✓ The unsaturated air reaches the condition of saturation, and the heat of vaporization of water is absorbed from the spray water itself so that the spray water gets cooled.  Psychometry and Air Conditioning Heating and Humidification ✓ During this process, the humidity ratio, the dry-bulb temperature, the wet-bulb temperature, the dew point temperature and the enthalpy of air increase while passing through hot spray. ✓ The relative humidity may increase or decrease. The spray water is to be heated before being pumped to the spray nozzles. ✓ The air enters at state 1 and leaves at state 2, as shown in figure.  Psychometry and Air Conditioning Heating and Humidification ✓ The mass balance for water is: Where, mw1, mw2 are the mass flow rates of water entering and leaving in kg/min, respectively, m is the mass flow rate of air in kg/min, and w1 and w2 are specific humidity of air entering and leaving respectively.  Psychometry and Air Conditioning AIR CONDITIONING ❑ UNITARY SYSTEM ✓ Such a unit is designed to be installed in or near the conditioned space. ✓ The components are contained in the unit. ✓ Unitary systems are standardized for certain applications, but minor modifications are possible to suit an application. ✓ Heating components are rarely included.  Psychometry and Air Conditioning AIR CONDITIONING Room Air Conditioners (Window AC) This is the simplest form of a unitary air-conditioning system, assembled inside a casing (see figure), and suitable for installation on a window or wall opening.  Psychometry and Air Conditioning AIR CONDITIONING Room Air Conditioners (Window AC) ✓ The unit thus consists of the refrigeration system, the control system (thermostat and selector switch), electrical protection system (motor overload switches and winding protection thermostat for the compressor motor), air- circulation system (fan motor, centrifugal evaporator blower and propeller fan for air cooled condenser) and ventilation (fresh air) damper and exhaust system. ✓ The refrigeration system consists of a hermetic compressor, forced air-cooled tinned condenser coil, finned cooling coil, capillary tube as throttling device and a refrigerant filter/drier.  Psychometry and Air Conditioning AIR CONDITIONING CENTRAL AIR CONDITIONING SYSTEMS ✓ A central air conditioning system can be used for single-zone (a zone consisting of a single room or group of rooms) or multi zone applications.  Psychometry and Air Conditioning AIR CONDITIONING CENTRAL AIR CONDITIONING SYSTEMS ✓ A single-zone air conditioning system has one thermostat that automatically controls one heating or cooling unit to maintain proper temperature in a zone comprising a single room or a group of rooms. ✓ A window air conditioner is an example of a single-zone air conditioning unit.  Psychometry and Air Conditioning AIR CONDITIONING CENTRAL AIR CONDITIONING SYSTEMS ✓ An air-handling unit (AHU) cools or heats the air, then distributes it to a single zone. ✓ The supply air fan is necessary to distribute air through the ductwork to the rooms. Cooling coil: It cools and dehumidifies the air and provides humidity control in summer. The reheat coil is optional and is used when the air temperature is to be maintained at the required level, especially in winter. In summer, it may remain idle.  Psychometry and Air Conditioning AIR CONDITIONING CENTRAL AIR CONDITIONING SYSTEMS Reheating coil: It heats the cooled air when the room heat gain is less than the maximum, thus providing humidity control in summer. The coil capacity is such that it satisfies the heating needs during winter. Ductwork: It is arranged so that the system takes in some outside ventilation air (OA), the rest being return air (RA) recirculated from the rooms. The equivalent amount of outside air must then be exhausted from the building. Dampers are provided to vary the rate of ventilation air as per the requirement of fresh air in the rooms. In some applications as in operating theatres, ventilation air can be 100%.  Psychometry and Air Conditioning AIR CONDITIONING CENTRAL AIR CONDITIONING SYSTEMS Return air fan: It takes the air from the rooms and distributes it through return air ducts back to the air conditioning unit or to the outdoors. In small systems with little or no return air ducts, the return air fan is not required because the supply fan can be used to draw in the return air. Preheat coil: The preheat coil may be located either in the outside air or the mixed airstream. It is required in cold climates (below freezing) to increase the temperature of air so that the chilled water-cooling coils do not freeze. It is optional in milder climates and when DX (dry expansion) cooling coils are used. Filters: The filters are required to clean the air.  Psychometry and Air Conditioning AIR CONDITIONING UNITARY VS. CENTRAL SYSTEMS ✓ In a unitary system, the refrigeration and air conditioning components are factory selected and assembled in a package. This includes refrigeration equipment, fan, coils, filters, dampers, and controls. ✓ A central system is one where all the components are separate. The engineer has to design and install the central plant; its suitable components are based on the air-conditioning load.  Psychometry and Air Conditioning AIR CONDITIONING UNITARY VS. CENTRAL SYSTEMS ✓ Unitary equipment is usually located in or close to the space to be conditioned. In contrast, the central equipment is usually remote from the space, and each component may or may not be remote, depending on the desirability. ✓ Central systems can be all-air, all-water, or air-water systems, and they are generally suitable for multi-zone units.  Psychometry and Air Conditioning AIR CONDITIONING HEAT PUMP UNIT ✓ A heat pump is an air conditioner where the refrigeration cycle can be reversed, producing heating instead of cooling indoors. ✓ They are also commonly called "reverse cycle air conditioners.” ✓ The heat pump is significantly more energy efficient than electric resistance heating.  Psychometry and Air Conditioning AIR CONDITIONING HEAT PUMP UNIT ✓ When the heat pump is in heating mode, the indoor evaporator coil switches roles and becomes the condenser coil, producing heat. The outdoor condenser unit also switches roles to serve as the evaporator and discharges cold air (colder than the ambient outdoor air).  Psychometry and Air Conditioning AIR CONDITIONING COMFORT AIR CONDITIONING ✓ Since most air-conditioning systems aim to provide a comfortable indoor environment, the system designer and operator should understand the factors that affect comfort. ❖ Body Heat Loss ✓ Heat is generated in the body due to the metabolism or digestion of food. This body heat is continually lost to its cooler surroundings. The factor that determines whether one feels hot or cold is the rate of loss of body heat. ✓ When the rate of heat loss is within certain limits, one feels comfortable. If the rate of heat loss is too much, one feels cold; if the rate is too low, one feels hot.  Psychometry and Air Conditioning AIR CONDITIONING COMFORT AIR CONDITIONING ❖ Body Heat Loss ✓ The processes by which the body loses heat to the surroundings are convection, radiation and evaporation. ✓ The rate of body heat loss is affected by five conditions: Air temperature Air humidity Air motion Temperature of surrounding objects  Psychometry and Air Conditioning AIR CONDITIONING COMFORT AIR CONDITIONING ❖ Body Heat Loss ✓ The system designer and operator can control comfort, primarily by adjusting three conditions: temperature, humidity and air motion. How are they adjusted to improve comfort? ✓ The indoor air temperature may be lowered to increase the body heat loss in summer while in winter it may be raised to decrease the body heat loss. ✓ Air motion may be raised to increase the body heat loss in summer and lowered to decrease the body heat loss in winter by convection.  Psychometry and Air Conditioning AIR CONDITIONING COMFORT AIR CONDITIONING ❖ Indoor Air Quality ✓ Another factor, air quality, refers to the degree of purity of the air. The level of air quality affects both comfort and health. Air quality is worsened by contaminants such as tobacco smoke and dust particles, biological microorganisms, and toxic gases. ✓ Cleaning devices such as filters may be used to remove particles. Adsorbent chemicals may be used to remove unwanted gases. Indoor contaminants can be diluted in concentration by introducing substantial outdoor air into the building. This procedure is called ventilation.  Psychometry and Air Conditioning AIR CONDITIONING COMFORT AIR CONDITIONING ❖ Effective Temperature ✓ Effective temperature (ET) is defined as that temperature of saturated air at which the subject would experience the same feeling of comfort as experienced in the actual unsaturated environment. ✓ Based on the concept of effective temperatures some comfort charts have been developed. To mention one, there is the Fanger's comfort chart. These may be referred to when a compromise in the inside design conditions is to be achieved.  Lab. Activity Lab. Activity No. No. 11 Plant and Livestock Microclimate: Please choose at least five plants and livestock and identify their ideal microclimate. Includes (but is not limited to): -Temperature (air and surrounding objects/material) -Relative Humidity -Wind velocity -Light -Precipitation -Soil Moisture and Temperature (specifically for plants) -Topography Use A4 bond paper with 6-10 pages only. (Printed)  Thank you. Thank you. Next Topic: Insulation and Vapor Barriers and Energy Considerations