Refrigeration & Air-Conditioning

Lesson 27. Psychrometric processes - sensible heating, sensible cooling, humidification, dehumidification and mixing of air.

Module 7. Psychrometry

Lesson 27
PSYCHROMATRIC PROCESSES-SENSIBLE HEATING, SENSIBLE COOLING, HUMIDIFICATION, DEHUMIDIFICATION AND MIXING OF AIR

27.1 Psychrometric Processes

It is necessary to carry out various processes on air in order to get the required quality of air. The quality of ambient air varies through out the year and by employing one or combination of processes, it is possible to get required conditions of the air.

27.2 Sensible Heating of Air

Heating of air without addition or subtraction of water vapour of the air is termed as sensible heating of the air. The sensible heating can be achieved by passing the air over heating coil like electric resistance heating coils or steam coils as shown in Fig. 27.1 The process is represented on the Psychrometric chart in Fig. 27.2

Fig. 27.1 Sensible heating of air

The efficiency of the heating coil is better with lower bypass factor. Lower by-pass factor of heating coil is desirable to achieve higher efficiency of the coil.

Example

An ambient air at 30 ºC dbt and 22 ºC wbt is heated to 180 ºC dbt in an indirect steam coil heater at the rate of 20 m³/min. Find the capacity of the heating coil in kW and the by-pass factor of the coil, if the surface temperature of the heating coil is 200 ºC.

Solution

27.3 Sensible Cooling of Air

Cooling of air without addition or subtraction of water vapour of the air is termed as sensible cooling of the air. Sensible cooling of air can be achieved by passing the air over a cooling coil like evaporating coil of the refrigeration cycle or secondary chilled water/brine coil as shown in Fig 27.2. Sensible cooling process is represented on psychrometric chart in Fig. 27.3.

Fig. 27.3 Sensible cooling of air

Fig. 27.4 Sensible cooling process on psychrometric chart

The capacity of cooling coil can be estimated as under.

capacity of cooling coil in TR = V_a /V_sp x h₁-h₂ /3.5

Where, V_a = flow rate of the air, m³/s

V_sp = specific volume of air, m³/kg dry air

h₁ = initial enthalpy of the air (kJ/kg of dry air)

h₂ = enthalpy of the cooled air (kJ/kg of dry air)

The efficiency of cooling coil is expressed as bypass factor (B) of the coil.

B =t₂-t₃ / t₁-t₃

The efficiency of the cooling coil is better with lower bypass factor. Lower by-pass factor is desirable to achieve higher efficiency of the coil.

Example

An ambient air at 40 ºC dbt and 30 % RH is cooled 25 ºC dbt & 21 ºC wbt by a cooling coil maintained at 20 ºC. The flow rate of the air is 10 m³/min. Find the capacity of the cooling coil in ton and the by-pass factor of the coil.

Solution

Corresponding to 40 ºC dbt and 30% RH, the psychrometric properties are,

h₁ = 76..5kJ/kg of dry air

ω₁ = 0.014 kg/kg of dry air

V_sp = 0.91 m³/kg dry air

= 1.005 X 25+ 0.014[2501+1.88X25]

= 60.79 kJ/kg dry air

27.4 Dehumidification of Air by Cooling

The removal of water vapour from the air is known as dehumidification of air. The dehumidification of air is achieved if the air is cooled below the dew point temperature of the air. Large scale dehumidification of air can be achieved by passing the air over a cooling coil maintained well below the dew point temperature of the air. The water vapour present in the air condenses over the surface of the cooling coil. The ambient air at point 1(t_1, ω_1, h₁) is passed over the cooling coil which is maintained at the surface temperature of t₄ (point 4). First, the air reaches to dew point temperature (t₂) and finally comes out from the cooling coil at point 3 (t_3, ω_3, h₃). The process is indicated in and the process is represented on psychrometric chart in (Fig.27.6)

As the cooling coil is not hundred per cent efficient, the condition of the air coming out of the coil will be at point 3. The bypass factor of the cooling coil is given by

Where, m_{a is the mass of dry air in kg per second in a mixture, passing over the coil.}

27.5 Adiabatic Chemical Dehumidification of the Air

When humid air is passed through the solid absorbent bed or through the spray of liquid absorbent, part of water vapour will be absorbed and the water vapour content of the air decreases as indicated in (Fig.27.7) The latent heat of the vapour is liberated resulting into increase in dbt of the air without any change in total enthalpy of the air. This method of dehumidification is economical for small size dehumidification of air and the change in humidity required is smaller.

27.6 Adiabatic Humidification of Air

Adiabatic humidification of air can be achieved by passing the air through the spray of water. When the air is passing through the spray of water, a part of water will be evaporated by using the sensible heat of the air. During this process, the specific humidity of the air increases and enthalpy of the air does not change. The process of adiabatic humidification is shown in Fig. 27.8. The latent heat required to evaporate the water is taken from the atmosphere of the area resulting into the decrease in the dbt of the air without any change in total enthalpy of the air. This method of humidification is economical for small size humidification of air and the change in humidity required is smaller. The process of adiabatic humidification is represented on psychrometric chart in (Fig.27.8)

The efficiency of the humidifier can be calculated as under.

27.7 Mixing of Air

When two air streams having different psychrometric property are mixed together, the quality of resultant air can be obtained by taking heat and moisture balance. The process of mixing of two air streams is indicated in Fig. 27.10 and it is represented on psychrometric chart in (Fig.27.9)