1 Refrigeration Technology wu wei-dong Chapter9. Psychrometry and Air Processes

2 Refrigeration Technology wu wei-dong  Composition and thermal properties of moist air Composition and thermal properties of moist air  Adiabatic saturation process and wet bulb temperature Adiabatic saturation process and wet bulb temperature  Psychrometric chart Psychrometric chart  Air handing processes Air handing processes  Heat and mass transfer between moist air and solid surface Heat and mass transfer between moist air and solid surface  REFERENCES REFERENCES Chapter9. Psychrometry and Air Processes

3 Refrigeration Technology wu wei-dong 9-1.Composition and thermal properties of moist air Chapter9. Psychrometry and Air Processes

4 Refrigeration Technology wu wei-dong 1. Atmospheric air  Atmospheric air is not completely dry but a mixture of dry air and water vapor.  In atmospheric air, the content water vapor varies from 0 to 3% by mass.  The processes of air-conditioning and food refrigeration often involve removing water from the air (dehumidifying), and adding water to the air (humidifying). Chapter9. Psychrometry and Air Processes

5 Refrigeration Technology wu wei-dong 2. The thermal parameters of moist air (1) Dry bulb temperature t  Dry bulb temperature is the temperature of the air, as measured by an ordinary thermometer.  The temperature of water vapor is the same as that of the dry air in moist air.  Such a thermometer is called a dry-bulb thermometer in psychrometry, because its bulb is dry. (2) Wet bulb temperature t WB :  Wet bulb temperature is thermodynamic adiabatic temperature in an adiabatic saturation process, and measured by a wet bulb thermometer.  The wet bulb temperature will be discussed in the next paragraph. Chapter9. Psychrometry and Air Processes

6 Refrigeration Technology wu wei-dong (3) Dew point temperature t DP :  When the unsaturated moist air is cooled at constant vapor pressure or at constant humidity ratio, to a temperature, the moist air becomes saturated and the condensation of moisture starts, this temperature is called dew point temperature of the moist air. (4) Relative humidity Ф:  Relative humidity is defined as the ratio of the mole fraction of the water vapor in a given moist air to the mole fraction of water vapor in a saturated moist air at the same temperature and the same atmospheric pressure.  Relative humidity is usually expressed in percentage (%).  From the ideal gas relations, relative humidity can be expressed as  ---- the mole fraction of the water vapor in moist air;  ---- mole fraction of water vapor in a saturated moist air at the same temperature; Chapter9. Psychrometry and Air Processes

7 Refrigeration Technology wu wei-dong (5) Degree of Saturation μ:  Degree of saturation is defined as the ratio of the humidity ratio of moist air w to the humidity ratio of saturated moist air w sat at the same temperature and atmosheric pressure. (6) Humidity ratio (Moisture Content) w:  The humidity ratio is the mass kg of water vapor interspersed in each kg of dry air.  It should be noted that the mass of water refers only to the moisture in actual vapor state, and not to any moisture in the liquid state, such as dew, frost, fog or rain.  The humididy ratio, like other several properties to be studied- enthalpy and specific volume-is based on 1kg of dry air. Chapter9. Psychrometry and Air Processes

8 Refrigeration Technology wu wei-dong (7) Specific Volume/Moist Volume v:  Specific volume of moist air v, m 3 /kg dry is defined as the total volume of the moist air (dry air and water vapor mixture) per kg of dry air. (8) Specific Enthalpy:  Specific enthalpy of moist air h (kJ/kg dry ) is defined as the total enthalpy of the dry air and water vapor mixture per kg of dry air.  Enthalpy values are always based on some datum plane.  Usually the zero value of the dry air is chosen as air at 0 ℃, and the zero value of the water vapor is the saturated liquid water at 0 ℃. Chapter9. Psychrometry and Air Processes

9 Refrigeration Technology wu wei-dong 9-2.Adiabatic saturation process and wet bulb temperature Chapter9. Psychrometry and Air Processes

10 Refrigeration Technology wu wei-dong 1. Adiabatic saturation process  An unsturated air, which has dry bulb temperature t 1, humidity ratio w 1 and enthalpy h 1, flows through a spray of water, as shown in Fig.9-1. Chapter9. Psychrometry and Air Processes  The spray can provide enough surface area so that the air leaves the spray chamber in equilibrium with the water, with respect to both the temperature and the vapor pressue.  In order to perpetuate the process, it is necessary to provide makeup water (w 2 - w 1 ) to compensate for the amount of water evaporated into the air.  The temperature of the makeup water is the same as that in the sump.  If the device is adiabatic, then the process is called adiabatic saturation process. Fig.9-1, An adiabatic saturation process

11 Refrigeration Technology wu wei-dong 2.Thermodynamic wet bulb temperature  In an adiabatic saturation process, the temperature of the water in the sump is called the thermodynamic wet bulb temperature t WB of the air, or simply the wet bulb temperature of the air.  An adiabatic saturation process is a constant wet bulb temperature process, the wet bulb temperatures of air at the inlet and the outlet are same, and they are equal to t WB.  That is Chapter9. Psychrometry and Air Processes

12 Refrigeration Technology wu wei-dong 3.Measurement of wet bulb temperature  A simple wet bulb thermometer used to measure the wet bulb temperature is based on the principle of the adiabatic saturation process.  It is an ordinary thermometer being wrapt with a cloth sleeve of wool or flannel, around its bulb.  The cloth sleeve should be clean and free of oil and thoroughly wet with clean fresh water.  The water in the cloth sleeve evaporates as the air flows at a high velocity (≥2.5m/sec).  The evaporation, which takes the heat from the thermometer bulb, lowers the temperature of the bulb.  The thermometer indicates approximately the wet bulb temperature t WB. Chapter9. Psychrometry and Air Processes

13 Refrigeration Technology wu wei-dong  The difference between the dry-bulb and wet-bulb temperatures is called the wet-bulb depression.  If the air is saturated, evaporation cannot take place, and the wet-bulb temperature is the same as the dry-bulb temperature; and the wet-bulb depression equals zero. Chapter9. Psychrometry and Air Processes

14 Refrigeration Technology wu wei-dong 9-3. Psychrometric chart Chapter9. Psychrometry and Air Processes

15 Refrigeration Technology wu wei-dong  A psychrometric chart graphically represents the thermodynamic properties of moist air, as shown in Fig.9-2.  It is very useful in presenting the air conditioning processes. Chapter9. Psychrometry and Air Processes  The psychrometric chart is bounded by two perpendicular axes and a curved line:  1) The horizontal ordinate axis represents the dry bulb temperature line t, in ℃ ;  2) The vertical ordinate axis represents the humidity ratio line w, in kg w /kg dry.air  3) The curved line shows the saturated air, it is corresponding to the relative humidity Ф=100%. Fig.9-2, Psychrometric Chart (B=101.3 kPa)

16 Refrigeration Technology wu wei-dong  The psychrometric chart incorporates seven parameters and properties.  They are dry bulb temperature t, relative humidity Ф, wet bulb temperature t WB, dew point temperature t DP, specific volume v, humidity ratio w and enthalpy h.  Dry-bulb temperature t is shown along the bottom axis of the psychrometric chart. The vertical lines extending upward from this axis are constant- temperature lines.  Relative humidity lines Ф are shown on the chart as curved lines that move upward to the left in 10% increments. The line representing saturated air ( Ф= 100% ) is the uppermost curved line on the chart. And the line of Ф = 0% is a horizontal ordinate axis itself. Chapter9. Psychrometry and Air Processes

17 Refrigeration Technology wu wei-dong  Wet-bulb temperature t WB : On the chart, the constant wet-bulb lines slope a little upward to the left, and the wet bulb temperature is read following a constant wet-bulb line from the state-point to the saturation line.  Dew point temperature t DP : This temperature is read by following a horizontal line from the state-point to the saturation line.  Specific volume v: It is shown from the constant-volume lines slanting upward to the left. Chapter9. Psychrometry and Air Processes

18 Refrigeration Technology wu wei-dong  Humidity ratio w: it is indicated along the right-hand axis of the chart.  Enthalpy h: It is read from where the constant enthalpy line crosses the diagonal scale above the saturation curve. The constant enthalpy lines, being slanted lines, are almost coincidental as the constant wet-bulb temperature lines.  Only two properties are needed to characterize the moist air because the point of intersection of any two properties lines defines the state- point of air on a psychrometric chart.  Once this point is located on the chart, the other air properties can be read directly. Chapter9. Psychrometry and Air Processes

19 Refrigeration Technology wu wei-dong 9-4. Air handing processes Chapter9. Psychrometry and Air Processes

20 Refrigeration Technology wu wei-dong 1. The specifications for human comfort   relative humidity:  air flow rate: Chapter9. Psychrometry and Air Processes

21 Refrigeration Technology wu wei-dong 2. Main air handing processes and their variations in properties (1) Sensible cooling along a cooling coil, or sensible heating along a heating coil Chapter9. Psychrometry and Air Processes  The sensible cooling happens when the air is cooled without altering the specific humidity.  The process is shown on the psychrometic chart by line 1a in Fig.9-4.  During this process, the relative humidity of the air will increase.  The sensible cooling can only take place under the condition when the temperature of the cooling coil is not below the dew point temperature of the air being processed. Fig.9-4, Main air handing processes and their variations in properties

22 Refrigeration Technology wu wei-dong Chapter9. Psychrometry and Air Processes Fig.9-4, Main air handing processes and their variations in properties  The sensible heating is similar to sensible cooling, but with the dry bulb temperature increasing (line 1b in Fig.9-4).  It should be noted that there should be no water within the heating system because the evaporation of the water will increase the specific humidity of the air.

23 Refrigeration Technology wu wei-dong (2) Adiabatic humidification and dehumidification using a humidifier or chemical dehumidifier Chapter9. Psychrometry and Air Processes Fig.9-4, Main air handing processes and their variations in properties  The adiabatic humidification occurs when water vapor, of which temperature is near the wet bulb temperature of the moist air, is added to the air (line 2a in Fig.9-4).  A humidifier performs this function by supplying the water vapor.  During the adiabatic humidification process along the constant wet bulb temperature line, the specific humidity of air will increase.  Reduction in dry bulb temperature will happen as the evaporated water will absorb heat.

24 Refrigeration Technology wu wei-dong Chapter9. Psychrometry and Air Processes Fig.9-4, Main air handing processes and their variations in properties  The adiabatic dehumidification is a reverse process of the humidification, as shown by line 2b in Fig.9-4.  This dehumidification is usually achieved by use of chemical agents, like absorbents or adsorbents.  In this process, the specific humidity decreases along the constant wet bulb temperature line, whilst the dry bulb temperature will increase, as a result of chemical reaction

25 Refrigeration Technology wu wei-dong (3) Cooling with change in specific humidity using conventional air conditioners Chapter9. Psychrometry and Air Processes Fig.9-4, Main air handing processes and their variations in properties  When the temperature of evaporator is lower than the dew point temperature of the entering air, some water vapor in the air will condense and drain off.  The dry bulb temperature and the specific humidity of the air will reduce.  This process can be shown by line 3 on the psychrometric chart in Fig.9-4.

26 Refrigeration Technology wu wei-dong 9-5. Heat and mass transfer between moist air and solid surface Chapter9. Psychrometry and Air Processes

27 Refrigeration Technology wu wei-dong  There are a lot of heat exchangers in refrigeration and air conditioning systems.  Moist air makes heat transfer and/or mass transfer with the solid surface of the heat exchangers.  If the solid surface is dry during the process, there is heat transfer only.  However, if the solid surface is wetted, there are both heat transfer and mass transfer. Chapter9. Psychrometry and Air Processes

28 Refrigeration Technology wu wei-dong 1. Sensible heat transfer between moist air and solid surface  The sensible heat transfer rate dq sensible, KW, from the wetted surface to the moist air is Chapter9. Psychrometry and Air Processes

29 Refrigeration Technology wu wei-dong 2. Mass transfer between moist air and solid surface  The mass transfer rate of water vapor dm, kg w /s, from the wetted surface to the moist air is Chapter9. Psychrometry and Air Processes

30 Refrigeration Technology wu wei-dong 3. Latent heat transfer between moist air and solid surface  The latent heat transfer rate dq latent, KW, from the wetted surface to the moist air is Chapter9. Psychrometry and Air Processes

31 Refrigeration Technology wu wei-dong 4. Lewis number and Lewis factor  Lewis number is defined as  The Lewis number Le N is a property of a mixture. For moist air, Lewis number Le N has values not far from 1.  In order to relate the heat transfer process, a new nondimensional, Lewis factor is introduced and is defined as  Lewis factor Le f is not a property of moist air, it is variable with the conditions of heat and mass transfer.  The value of Lewis factor Le f is considered in the region of Chapter9. Psychrometry and Air Processes

32 Refrigeration Technology wu wei-dong 5. Total heat transfer between moist air and solid surface  The total heat transfer rate, sensible and latent heat transfer, from the wetted surface to the moist air.  Applying the Lewis factor Le f =1.0 Chapter9. Psychrometry and Air Processes

33 Refrigeration Technology wu wei-dong  The Eq. above can be written as  For dry air, specific heat  For moist air, specific heat  For the most cases of air conditioning  Therefore Chapter9. Psychrometry and Air Processes

34 Refrigeration Technology wu wei-dong  Therefore the concept of enthalpy potential is a very useful one in quantifying the total heat transfer in those processes where there is direct contact between air and water.  If t a > t s, the sensible heat is from the moist air to the solid surface;  If w a >w s, the mass and latent heat is from the moist air to the solid surface;  If h a >h s or t a, w a >t s, w a, the total heat is from the moist air to the solid surface. Chapter9. Psychrometry and Air Processes

35 Refrigeration Technology wu wei-dong REFERENCES 1.Stoecker W.F., Jones J.W., Refrigeration and air conditioning, 2nd Edition, McGraw-Hill Book Company, New York, USA, ASHRAE, ASHRAE Handbook-Fundamental, New York, USA, Eckert E.R.G., Drake R.M., Analysis of heat and mass transfer, McGraw-Hill Ltd, New York, USA, Kloppers J.C., Kroger D.G., The Lewis factor and its influence on the performance prediction of wet-cooling towers, International Journal of Thermal Science, (44), , 2005 Chapter9. Psychrometry and Air Processes