The common Psychometric processes which are usually encountered in practice and which may be easily solved with the help of a psychometric chart are given below and are indicated in Fig. 37.6.
1. Simple heat transfer process–
(a) Sensible cooling process
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(b) Sensible heating process.
2. Simple Humidification process at same DBT.
3. Simple dehumidification process at same DBT.
4. Heat transfer processes with change in humidity.
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(a) Cooling and dehumidification
(b) Heating and humidification
(c) Heating and dehumidification.
(d) Cooling and humidification.
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5. Adiabatic mixing of two samples of air.
These processes will now be explained individually:
1. Simple Heat Transfer Process:
(a) Sensible Cooling:
This process involves cooling of air at constant moisture content. Process 1-2 is a horizontal line from right to left. Evidently, such cooling of air has to be carried on by bringing it into contact with a body or a surface temperature of which is not below the dew point temperature of air at 1.
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Heat removed = h2-h1 kJ/kg of dry air.
Generally all the air passing through the air duct or flow passage does come in contact with the coil surface and the temperature of the air coming out of the passage is not the temperature of the coil surface. With reference to Fig. 37.6 (a), let tc be the temperature of the coil surface. Some of the air bypasses the coil surface and this bypassed air at t1 mixes with the other part of air at temperature tc and the final temperature of the mixture is, say, t2.
The ratio of the bypassed air to the total air is called the bypass factor (BF).
Given the bypass factor and the coil surface temperature, actual temperature of air can be found by using the equation.
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(b) Sensible Heating Process:
This process involves of heating of air without the change of humidity or humidity content.
Heating process line is a straight line from left to right Fig. 37.6 (b)
Heat supplied/kg of air = h2 – h1 kJ/kg dry air.
For this heating process,
The amount of bypass depends upon:
(a) Velocity of air over the coil.
(b) Number of rows of coil pipes in the direction of flow.
(c) In case of a coil with fins attached, the bypass factor depends on the number of fins per unit length on the coil.
2. Simple Humidification Process at the same DBT:
Adding moisture at constant dry bulb temperature can be represented by a vertical line on a psychometric chart.
Pure humidifying process is possible only when heat of vaporisation is supplied from a source other that air without affecting the dry bulb temperature. This happens when water at the dry-bulb temperature of the entering air is sprayed in the chamber whose temperature is maintained at the air DB temperature.
Heat supplied = h2 – h1 kJ/kg dry air = (w2 – w1) x Latent heat
Moisture added = w2 – w1 kg/kg dry air.
3. Simple Dehumidification Process at same DBT:
Pure dehumidification occurs without the change of dry- bulb temperature of air, when air is passed over chemical absorbing moisture and heat evolved, when moisture is absorbed, is removed immediately by the surrounded walls. These walls are maintained at the dry-bulb temperature of air. The chemical is also maintained at the dry-bulb temperature of air. The process of pure dehumidification is shown in Fig. 37.6(d).
Heat removed = h1 – h2 = (w1 – w2) x Latent heat.
Pure or simple humidification and dehumidification processes are not found in practice and are always accompanied by heating and cooling.
4. Heat Transfer Processes with Change in Humidity:
(a) Cooling and Dehumidification Process:
This process involves lowering of both temperature and specific humidity, and may be carried on by bringing the moist air into contact with a chilled body or surface having its temperature lower than the dew point temperature of the air.
The cooling and dehumidification process of moist air may be carried on by any one of the methods given below:
1. By causing the air to flow over a chilled surface, below dew point temperature, which may be cooled by circulating chilled water or by directly evaporating the refrigerant of a refrigeration system, within the coil.
2. By causing the air to flow through an air washer spraying chilled water below dew point temperature.
These processes have practical applications in summer air conditioning. Since the actual process is difficult to trace on the chart, only end states are shown distinctly and joined by dotted curve. But possible processes can be shown as – (1-2) is the sensible cooling up to the dew point temperature and then cooling of air below dew point temperature as shown by (2-3). t3 is the coil surface temperature and if the coil is having a bypass factor then the actual state of air may be given by point (4). t3 is the apparatus dew point temperature ADP and the bypass factor is given by-
(b) Heating and Humidification Process:
This process is the reverse of that discussed in 4 (a) above. In this process the specific humidity and the dry bulb temperature of air are increased.
This process is carried on by blowing the air through an air washer spraying hot water. The process has practical application in winter air conditioning. During this process (i) specific humidity, dry bulb temperature, wet bulb temperature, dew point temperature and enthalpy increase and (ii) RH may increase or decrease.
The final state 2 of the air may also be achieved by a combination of sensible heating 1-3 and then by sensible humidification process 3-2.
Heating and humidification can also be achieved by injecting into air, the steam. This process is generally used in textile industry where high humidity is to be maintained. Rise in dry bulb temperature is generally very little. Instead of hot water, steam will be injected.
(c) Heating and Dehumidification Process:
There are certain chemical substances like silica gel and activated alumina which have an affinity for moisture. If air is passed over such chemicals it may be dehumidified. Usually in doing so the moisture is condensed and gives up its latent heat, raising the dry bulb temperature of air.
This process is reverse of adiabatic saturation process.
This is shown in Fig. 37.6(g). In figure, 1-2 adiabatic saturation process 1-3 adiabatic dehumidification process.
The path followed during this (adiabatic dehumidification) process is along the constant wet bulb temperature line or constant enthalpy line.
Actually there are two types of chemicals used for chemical dehumification.
They are:
1. Absorbents:
They include water solutions or brines of calcium chloride, lithium chloride, lithium bromide and ethylene glycol. Generally these are used in absorption refrigeration systems.
2. Adsorbents:
These are the substances in solid state and while absorbing moisture from air, they do not change chemically and physically. These are silica gel and activate alumina (porous amorphous form of aluminium dioxide).
(d) Cooling and Humidification (Adiabatic Humidification):
This process is explained in Sec. 37.2.9 and is a constant wet bulb temperature process. Air passing through a spray chamber with re-circulated water follows the process line 1-2 in Fig. 37.6(g). Whereby it cools due to the evaporation of water. Such cooling of air shown schematically in Fig. 37.6(h), will accompany humidification following only one path i.e., constant wet-bulb temperature line.
This principle is made use of in desert coolers for cooling buildings in hot and dry climate so as to make them comfortable. Since the cooling cannot be achieved below the wet bulb temperature of the outside or incoming air, at which temperature the leaving air will be saturated, this technique of cooling has only limited use. Hence the system becomes ineffective in monsoon season.
Perfect humidification of air is not possible in actual practice due to inefficiency of spray chamber. Therefore, the final condition of air at outlet is represented by state 3 on the line 1-2 as shown in Fig. 37.6(i). The effectiveness or the humidifying efficiency of the spray chamber is given by-
5. Adiabatic Mixing of Air Streams:
Very often air having different conditions are mixed in the air conditioning system, such as the mixing of fresh air and return or re-circulated air before it enters the process plant. The psychometric chart can be used to find the resulting condition of the mixture.
Figure 37.6(j) shows the schematic diagram showing two streams of air mixing together.
Let the two streams of masses m1 and m2 humidity rations w1 and w2 and enthalpies h1 and h2 respectively, mix together without any heat exchange with surroundings and also exchange of moisture to give a stream of mass m3 = m1 + m2 humidity w3 = w1 + w2 and enthalpy h3 = h1 – h2.
The final condition of the mixture can be found with the help of chart as follows:
First determine the percentage of air in each stream, in the mixture. Let in stream one, this is x%. Therefore, (100 – x) is the percentage of air mass in other stream.
Then locate the two points 1 and 2 from the given conditions of two streams. Join 1 and 2. Next step is very important. Multiply the dry bulb temperature of each air by its percentage in the mixture. Let this be x.t1. This is the contribution of stream (1) DB degrees to the mixture. Similarly for second stream (100 -x) t2 will be DB degrees contributing in the mixture.
x t1 + (100 – x) t2 = 100 x t3
t3 can be found.
Draw a vertical at t3 to meet the line 1-2 in 3. Then all other properties can be found from the chart.