Faculty of Architecture and Planning Thammasat University AR/IA/UP 241 Lecture 5: Psychrometrics Author: Asst. Prof. Chalermwat Tantasavasdi 1. Definition of Psychrometric Chart The word psychrometry is the combination of the words psychro (Greek psychros = cold) and metry (= measurement). Therefore it directly means measurement of coolness. In the earth atmosphere, the components of air include two major parts: gas of different types and water vapor. As a result, a psychrometric chart is used for quantitative study of the relationship between air and its water vapor content. 1
2. Components of Psychrometric Chart The psychrometric chart is a very useful tool for thermal science since it describes the relationship of a number of important parameters. These include dry bulb temperature, wet bulb temperature, absolute humidity, relative humidity, specific volume and enthalpy. This section explains each of the parameters one by one. Dry Bulb Temperature (DBT, T d ), C, F DBT is the air temperature measured by a regular thermometer. It represents the X-axis on the psychrometric chart. Wet Bulb Temperature (WBT, T w ), C, F WBT is the air temperature measured by a thermometer covered with wet bulb. It is generally lower then DBT because water vapor from the bulb evaporates, taking the heat out off the thermometer. Dry air allows water vapor to evaporate more quickly than wet air. Therefore, the dryer the air is, the lower WBT will be. Hence, WBT can hint for the moisture content in the air. Absolute Humidity (AH, ω, Humidity Ratio), g/kg dry air, lb/lb dry air AH is the actual moisture content in the air, presenting the Y-axis on the psychrometric chart. It is useful for the purpose of calculation but the terms are not easy to understand. Relative Humidity (RH, φ), % RH is the moisture content in the air as a percentage of its saturation humidity, or can be expressed as the following equation. ω = 100% φ (1) ω s In other words, RH explains how wet the air is in relation to its ability to contain moisture. The ability of air to contain moisture varies according to its temperature. Therefore, saturation point also varies along with its temperature. RH disregards these varieties and thus can easily be understood but not very useful for calculations. Specific Volume (SV, V) = 1/ρ, m 3 /kg dry air, ft 3 /kg dry air SV is the inversion of the air density. Warm air is lighter, thus occupying more volume than cool air. Hence, it has more SV. Enthalpy (H, h, i), KJ/Kg dry air, BTU/lb dry air H is the heat content of the air mixture in relation to the content at freezing temperature. Psychrometric chart is generally used for the calculation of heating or cooling of air from one condition to another. The difference of enthalpy between two air conditions (Δh) therefore presents the amount of heat necessary to be put in or taken out. 2
3. Condensation Humidity could cause problems in construction, especially for humid climatic areas. This section discusses an important humidity problem called condensation. Topics include temperature gradient in construction, dew point temperature, and condensation problem in building construction. Temperature gradient in construction Temperature gradient in construction explains the temperature distribution at each point in a section of construction. It can be calculated using the assumption that the same amount of heat flux would equally transfer through each part of the section. In general construction, temperature drops according to its sectional R-value. Improving thermal resistance of a construction by adding insulation material changes the temperature gradient profile. Although insulation material improves the thermal performance, it could impose other problem regarding moisture content in the construction. Dew point temperature (T d ) T d refers to the temperature where condensation starts. It depends on the temperature and moisture content. For example, at high humidity condition, the condensation would occur more easily than low humidity condition; T d is therefore high and stays close to DBT. Condensation problem in building construction Condensation could occur in a building construction if the temperature within its construction falls below T d. Condensation in some insulation materials, e.g. fiberglass, could cause further problems regarding growing fungi within construction. Vapor barrier, e.g. foil, could be installed to protect the moisture intrusion to the insulation material. Condensation could be visible on transparent material such as glass. Notice how it occurs on a car windshield and displaying freezer. This problem can be alleviated by allowing dry air to flow through the glass surface, decreasing its T d. 4. Thermal Comfort A human body tries to maintain its temperature at 37 C. However, the body metabolism via activities requires heat transfer from the body to the environment to keep its body temperature. This section discusses how a human body reacts to its surrounding environments to keep a steady comfort condition. Topics include heat balance of a human body, environmental factors, and other factors. Heat balance of a human body A human body exchanges heat between its body and surrounding environments in order to maintain a steady condition. Heat transfer methods include conduction, convection, radiation, and evaporation. 3
Conduction Conduction occurs where a part of human body directly touches a substance with different surface temperature to human skin, e.g. from the feet to the ground or from the butt to the seat. Convection When the air temperature is lower than that of the human skin, the air around the body takes the heat out off the body by means of convection. Radiation Any surfaces with different temperatures to human skin generate heat transfer by radiation. It can be heat gain to the human body if the surface has a higher temperature. On the contrary, it can be heat loss from the human body if the surface has a lower temperature. Evaporation Sweat and exhalation mean water vapor from human body evaporates. Such process takes the heat out off the human body. Environmental factors Environmental factors that influence heat transfer of a human body include temperature, humidity, air movement and mean radiant temperature (MRT). The effect of each factor on each type of heat transfer method varies according to the following table. Heat Transfer Method Environmental Factors Conduction Convection Radiation Evaporation Temperature Humidity Air Movement Mean Radiant Temperature Other factors Aside from environmental factors, there are other two factors that directly affect the heat transfer of a human body. They are human activities and clothing. Activities The level of activeness of human influences the metabolism rate (MET). This results in the amount of heat transfer needed for human body to maintain its temperature. Clothing Each type of clothing has different thermal resistance, known as Clo-value. It affects the resistance in heat transfer equation. 4
5. Thermal Comfort Standards To maintain a human body temperature, heat transfer in and out of the body has to be balanced. Scientists believe that human has certain comfort conditions and try to create standards for thermal comfort conditions. However, numerous studies show discrepancies for such conditions. This section discusses major studies regarding thermal comfort standards including ASHRAE comfort zone, variation of comfort zone, and bio-climatic chart. ASHRAE comfort zone (ASHRAE Standard-55) American Society of Heating Refrigerating and Air-conditioning Engineers (ASHRAE) have continuously been studying human thermal comfort and create standards for people, known as ASHRAE Standard-55. The latest version was created in 2004. The standard gives ranges of comfortable temperature (19.5-31.5 C) and humidity (0-12 g/kg dry air) according to a set of air velocities (0.2-1.5 m/s). The conditions apply only to situation where there is a system to control humidity, e.g. air-condition. Variation of comfort zone Other groups of scientists believe that human can acclimatize to different types of climate; e.g. can tolerate higher temperature and humidity in hot-humid climate. The concept of acclimatization also applies to modes of air-conditioning; e.g. people who are used to naturally ventilated environment should have the comfort zone different from those who are used to air-conditioned environment. Khedari et al., for example, study comfort zone for naturally ventilated space in Thailand and find the upper ranges of temperatures (27.0-36.3 C) and humidity (50-80%) higher than those in ASHRAE Standard, according to the same set of air velocities (0.2-3.0 m/s). Air Velocity (m/s) Acceptable Temperature Range (C) 0.2 27.0-29.5 0.5 28.5-30.8 1.0 29.5-32.5 1.5 31.0-33.8 2.0 31.2-36.0 3.0 31.6-36.3 Bio-climatic chart Victor Ogyay first invented the bio-climatic chart showing relationship between RH and DBT in 1960 s as an alternative method for thermal comfort study. The chart incorporates the effects of air movement and radiation. Although it is easier to understand than psychrometric chart, calculations are more difficult because it does not provide AH, V, and H. 5
6. Conditioning Processes In general, the ambient environments might not be within the thermal comfort standards. The air temperature, for example, could be too warm or too cool. Therefore, the air needs to go through a conditioning process to shift its condition to be within a comfort standard. The processes include heating, cooling, evaporative cooling, and desiccant dehumidification. This section explores each of the methods. Heating Heating is a straightforward process that gives the heat to the air. A general equation for heat given to/taken away from the air can be express as: (2) Q = m Δh where Q = Rate of heat given/taken away to the air (kw) m = Mass flow rate of the air (kg/s) Δ h = Enthalpy difference between the air before and after conditioning (kj/kg) In the heating process, since there is no moisture content added or extracted from the air, equation (2) can also be expressed as: (3) Q m c p ΔT = cooling heating Cooling Comparing to heating, cooling is a more difficult task and more energy consuming because it needs three sub-processes: Cooling of air to its saturation Cooling and extracting moisture along its saturation line Reheating to the required temperature 6
The energy required for each of the three sub-processes can be computed using equation (2). The cooling and reheating parts could also use equation (3) since there is no moisture content involved. The heat involving the process of adding/extracting heat can also be referred to as Sensible heat while that involving the process of adding/distracting moisture can be named Latent heat. Evaporative (Adiabatic) cooling The process of naturally adding water vapor to the air is called evaporative cooling. The temperature reduces while the humidity increases. Since there is no energy involved in the process, it can be also called adiabatic cooling where the conditions shift along the enthalpy line. It is a very effective method for hot-dry climate due to its low humidity air conditions. For hot-humid climate, however, this method is not as effective but still can reduce some degrees of temperature. Desiccant (Adiabatic) dehumidification Dehumidification is the opposite process to evaporative cooling using moisture absorption material or chemical substance, e.g. silica gel. Like evaporative cooling, the process does not involve energy change and therefore can be called adiabatic dehumidification. The conditions also shift along the enthalpy line but on the opposite direction to the evaporative cooling. It is not practical yet since the substance becomes saturated and needs drying process. There is numerous ongoing research and development to create a workable system. evaporative cooling desiccant dehumidification 7
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