Psychrometric Processes

Air – Conditioning Processes Air-conditioning processes include: Simple heating (raising the temperature), Simple cooling (lowering the temperature) Humidifying (adding moisture), and Dehumidifying (removing moisture).

Psychrometric Chart : The properties of atmospheric air at a specified total pressure are presented in the form of easily readable charts called Psycrometric Chart. The dry – bulb temperature are shown on the horizontal axis. The specific humidity is shown on the vertical axis.

The psychrometric chart Wet Bulb & Saturation Curve G Enthalpy F D Dew Point E A Absolute Humidity Axis D Relative Humidity C Dry Bulb Axis B Specific Volume

For a saturated air, the dry – bulb, wet – bulb and dew – point temperature are identical.

Schematic of a psychrometric chart for a given barometric pressure Constant RH% Lines Schematic of a psychrometric chart for a given barometric pressure

Cpm = Sp. Heat of moist air a) 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: Cpm = Sp. Heat of moist air

Define BPF. It’s relationship with effectiveness of coil. What is heating capacity of coil?

b) 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 remain constant, 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. The heat transfer rate during this process is given by:

Simple Heating and Cooling (ω = constant) There is no change in the absolute humidity of the air-vapor mixture. Cooling occurs from right to left. Heating occurs from left to right. There is a change in the sensible heat of the air-vapor mixture. Heat must be added or subtracted to cause the temperature change. Cooling Heating Dry Bulb Temperature

Humidification and dehumidification Process: 1. Show it on Psychrometric Chart. 2. How to achieve these processes? 3. How to determine LH added in these processes?

QL = Latent Heat Transfer

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.

BPF =( Ts – TADP) / (Tr – TADP ) Summer cooling and dehumidification processes: 1- All outside air , 2- All return air , 3- Mixing of fresh air with return air : as shown in the figure below. Sensible Heat Ratio ( SHR ) = Sensible heat/Total heat SHR = Qs /(Qs+Ql )ٍ By Pass Factor (BPF) : Is the factor that determine the quantity of air that by pass the cooling coil with out contacting its surfaces . BPF =( Ts – TADP) / (Tr – TADP ) Where TADP is the apparatus dew point temperature of the cooling coil .

Evaporative Cooling/ Adiabatic Cooling Evaporative cooling is based on a simple principle: As water evaporates, the latent heat of vaporization is absorbed from the water body and the surrounding air. And both the water and the air are cooled during the process.

During evaporative cooling the wet bulb temperature remains constant.

Heating with Humidification

d) Chemical Dehumidification Process

Adiabatic Mixing of Air streams When two airstreams at two different states (states 1 and 2) are mixed adiabatically, the state of the mixture (state 3) will lie on the straight line connecting states 1 and 2 on the psychrometric chart, and the ratio of the distances 2-3 and 3-1 is equal to the ratio of flow rates and .

H3 = m1h1 + m2h2 ---------------------------------; Similarly for v3 and w3 m1+m2

e) Mixing of Two Air Streams without condensation

Air Washers:

Air Washers: An air washer is a device for conditioning air. As shown in Fig., in an air washer air comes in direct contact with a spray of water and there will be an exchange of heat and mass (water vapour) between air and water. The outlet condition of air depends upon the temperature of water sprayed in the air washer. Hence, by controlling the water temperature externally, it is possible to control the outlet conditions of air, which then can be used for air conditioning purposes.

Cooling and dehumidification: tw < tDPT Cooling and dehumidification: tw < tDPT. b) Adiabatic saturation: tw = tWBT. c) Cooling and humidification: tDPT < tw < tWBT. d) Cooling and humidification: tWBT < tw < tDBT. e) Heating and humidification: tw > tDBT.