Psychrometric Properties & Processes

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Introduction:

Psychrometry :The Science which deals with the study of the behavior of air and water vapour mixture is known as Psychrometry. The properties of water vapour and air mixture are known as Psychrometric Properties.

In the design and analysis of air conditioning plants, the fundamental requirement is to identify the various processes being performed on air. Once identified, the processes can be analyzed by applying the laws of conservation of mass and energy.

The+psychrometric+chart+has+seven+lines. - Psychrometric Properties & Processes

All these processes can be plotted easily on a psychrometric chart. This is very useful for quick visualization and also for identifying the changes taking place in important properties such as temperature, humidity ratio, enthalpy etc. The important processes that air undergoes in a typical air conditioning plant are discussed below .

Important psychrometric processes:

 

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a) Sensible cooling:

During this process, the moisture content of air remains constant but its temperature decreases as it flows over a cooling coil. For moisture content to remainconstant, the surface of the cooling coil should be dry and its surface temperature should be greater than the dew point temperature of air. If the cooling coil is 100% effective, then the exit temperature of air will be equal to the coil temperature. However, in practice, the exit air temperature will be higher than the cooling coil temperature. Figure shows the sensible cooling process O-A on a psychrometric chart. The heat transfer rate during this process is given by:

Qc = ma(ho – ha)

b) Sensible heating (Process O-B):

During this process, the moisture content of air remains constant and its temperature increases as it flows over a heating coil. The heat transfer rate during this process is given by:

Qh = ma(hB – hO)

where cpm is the humid specific heat (≈1.0216 kJ/kg dry air) and ma is the mass flow rate of dry air (kg/s). Figure 28.2 shows the sensible heating process on a psychrometric chart.

c) Cooling and dehumidification (Process O-C):

When moist air is cooled below its dew-point by bringing it in contact with a cold surface as shown in Fig., some of the water vapor in the air condenses and leaves the air stream as liquid, as a result both the temperature and humidity ratio of air decreases as shown. This is the process air undergoes in a typical air conditioning system. Although the actual process path will vary depending upon the type of cold surface, the surface temperature, and flow conditions, for simplicity the process line is assumed to be a straight line. The heat and mass transfer rates can be expressed in terms of the initial and final conditions by applying the conservation of mass and conservation of energy equations as given below: By applying mass balance for the water:

ma.WO = ma.WC + mW

d) Heating and Humidification (Process O-D):

During winter it is essential to heat and humidify the room air for comfort. As shown in Fig.., this is normally done by first sensibly heating the air and then adding water vapour to the air stream through steam nozzles as shown in the figure.

Mass balance of water vapor for the control volume yields the rate at which steam has to be added, i.e., mw:

mW = ma(WD – Wo)

where ma is the mass flow rate of dry air

e) Cooling & humidification (Process O-E): As the name implies, during this process, the air temperature drops and its humidity increases. This process is shown in Fig.28.6. As shown in the figure, this can be achieved by spraying cool water in the air stream. The temperature of water should be lower than the dry-bulb temperature of air but higher than its dew-point temperature to avoid condensation (TDPT

It can be seen that during this process there is sensible heat transfer from air to water and latent heat transfer from water to air. Hence, the total heat transfer depends upon the water temperature. If the temperature of the water sprayed is equal to the wetbulb temperature of air, then the net transfer rate will be zero as the sensible heat transfer from air to water will be equal to latent heat transfer from water to air. If the water temperature is greater than WBT, then there will be a net heat transfer from water to air. If the water temperature is less than WBT, then the net heat transfer will be from air to water. Under a special case when the spray water is entirely recirculated and is neither heated nor cooled, the system is perfectly insulated and the make-up water is supplied at WBT, then at steady-state, the air undergoes an adiabatic saturation process, during which its WBT remains constant. This is the process of adiabatic saturation discussed in Chapter 27. The process of cooling and humidification is encountered in a wide variety of devices such as evaporative coolers, cooling towers etc.

f) Heating and de-humidification (Process O-F):

This process can be achieved by using a hygroscopic material, which absorbs or adsorbs the water vapor from the moisture. If this process is thermally isolated, then the enthalpy of air remains constant, as a result the temperature of air increases as its moisture content decreases as shown in Fig.28.7. This hygroscopic material can be a solid or a liquid. In general, the absorption of water by the hygroscopic material is an exothermic reaction, as a result heat is released during this process, which is transferred to air and the enthalpy of air increases.

g) Mixing of air streams:

Mixing of air streams at different states is commonly encountered in many processes, including in air conditioning. Depending upon the state of the individual streams, the mixing process can take place with or without condensation of moisture.

Psychrometric Properties :

09 Properties of moist air 10 - Psychrometric Properties & Processespsychrometry 4 638 - Psychrometric Properties & Processes

  • Dry Air
The pure dry air is a mixture of number of gases such as nitrogen, oxygen, and hydrogen and carbon dioxide. Density of air is 1.293kg/m3 at 1.01325 bar 273oK. Pure dry air does not exist in nature as it contains some impurities and some moisture all the time.
  • Moist Air
It is a mixture of dry air and water vapour. The amount of water vapourpresent in air depends on the pressure and temperature of the mixture.
  • Saturated Air
It is the mixture of dry air and water vapour when air has diffused maximum amount of water vapour into it. This means air has reached its
moisture carrying capacity to the fullest and no more addition is possible.
  • Degree of Saturation (μ)
It is the ratio of actual mass of water in a unit mass of dry air to the mass of water vapour in unit of saturated air at same temperature and pressure.
  • Specific Humidity (w)
It is the mass of water vapour present in 1kg of dry air and is expressed in terms of gram per kg of dry air.
  • Humidity Ratio
It is the same as specific humidity, i.e., mass of water vapour, present in unit mass of dry air.
  • Absolute Humidity
It is the mass of water vapour present in 1m3 of dry air and is expressed in terms of grams/m3 of dry air. It can be expressed in terms of grains per cubic meter of dry air. 1kg of water vapour = 15,430 grains.
  • Relative Humidity
It is the ratio of actual mass of water vapour in a given volume of moist air to the mass ofwater vapour in the same volume of saturated air at the same temperature and pressure.
  • Specific Volume (v)
It is defined as the ratio of volume per unit mass of moist air.
  • Dry Bulb Temperature (DBT) or Td
It is the temperature of air recorded by a thermometer when its bulb is surrounded by awet cloth exposed to air.
  • Wet Bulb Depression (WBD)
It is the difference between dry bulb temperature and wet bulb temperature at any point.
  • Enthalpy
Enthalpy of moist air is equal to enthalpy of dry air plus enthalpy of water vapour associated with dry air
  • Dew Point Temperature (DPT) or Tdp
It is the temperature of air recorded by a thermometer when water vapour in air begins to condense or dew is formed.
  • Psychrometer
It is an instrument to measure DBT and WBT of moist air.
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