Experiment 8

Cooling and de-humidification of air

1. Objective

The objectives of this experiment are:

1. To understand the process of cooling and de-humidification of air using the given apparatus and to estimate the cooling rate.

2. Introduction

Moist air can be divided into two components. They are the moisture part and the dry air part. The temperature of air that is measured when the bulb of a thermometer is dry is called as the dry bulb temperature. The quantity of moisture available in one kilogram of dry air is called as the specific humidity of the air. Further, the quality of air for air conditioning applications is indicated by two parameters namely the dry bulb temperature and relative humidity. Let us understand the cooling and dehumidification process in more details. When the air comes in contact with the cooling coil that is maintained at the temperature below its dew point temperature, its DB temperature starts reducing. The process of cooling continues and at some point it reaches the value of dew point temperature of the air. At this point the water vapor within the air starts getting converted into the dew particles due to which the dew is formed on the surface of the cooling and the moisture content of the air reduces thereby reducing its humidity level. Thus when the air is cooled below its dew point temperature, there is cooling as well as dehumidification of air.

The cooling and dehumidification process is most widely used air conditioning application. It is used in all types of window, split, packaged and central air conditioning systems for producing the comfort conditions inside the space to be cooled. In the window and split air conditioners the evaporator coil or cooling coil is maintained at temperature lower than the dew point temperature of the room air or the atmospheric air by the cool refrigerant passing through it. When the room air passes over this coil its DB temperature reduces and at the same time moisture is also removed since the air is cooled below its DP temperature. The dew formed on the cooling coil is removed out by small tubing. In the central air conditioning systems the cooling coil is cooled by the refrigerant or the chilled water. When the room air passes over this coil, it gets cooled and dehumidified.

Human comfort requires that both the parameters are maintained. Different methods are adopted for maintaining these conditions in practice. Cooling is normally done by using a refrigeration system evaporator coil either directly or with the help of an intermediate fluid. De-humidification is achieved by the use of cooling coils at temperatures below the dew point temperature of the air sample. Heating is done by using heating coils with electric heaters or coils heated with steam or hot water. Humidification is done by spraying hot water or steam into the

stream of air. The Hilton Air-conditioning experimental unit uses the above methods to change the conditions of air.

The psychrometric properties like dry bulb temperature, wet bulb temperature, dew point temperature, relative humidity and humidity ratio are defined.

The purpose of the experiment is to apply the air conditioning theory to the air as it goes through the air conditioning unit. The measured air properties are compared with the properties obtained from the psychrometric chart. It is also hoped that the student gets familiar, through this experiment, with some air conditioning processes. In the present experiment we will deal with cooling below the dew point, resulting in a drop in dry-bulb temperature and a reduction in the level of humidity.

3. Equipment

This experiment will be conducted on the computerized Hilton Air Conditioning Unit. This test rig facilitates the study of different air-conditioning processes either separately or in some combinations. A boiler, whose source of heat is an electric heating coil, generates steam. Data can be acquired either manually through an analog read-out or by the computer through an analog to digital converter. Data can be stored on disk for later processing. Data can be acquired either manually through an analog read-out or by the computer through an analog to digital converter. Data can be stored on disk for later processing. A boiler, whose source of heat is an electric heating coil, generates steam.

Figure 1, Schematic of cooling and dehumidification apparatus.

Table 1, Definition of apparatus components.

4. Pre-Lab

What are the methods available to find the humidity of air?

What are sensible heat and Latent heat loads?

5. Operating Instructions and Procedure

Start up the test rig as recommended in the rig manual. This is followed by running the fan at a certain speed at an uniform rate. The air carries the humidity along with it which can be measured using the phychrometers provided. Set the equipment to supply a given quantity of fresh air. This can be varied for getting different sets of readings. Adjust the boiler to provide a certain quantity of humidity by setting the kilowatts of energy supplied to the boiler. The humidification process starts and should achieve the steady state. The vapor-compression system (the cooling equipment) is used for cooling/dehumidification. The de-humidification process occurs due to the condensation of moisture on the evaporator tubes when the air reaches the dew point temperature. Some time is given until steady conditions are obtained as observed by the steadiness of various temperature readings. This usually takes about 25-35 minutes. Follow the on-screen menus and store the readings of all temperature and pressure differential sensors on hard disk and get a printout.

In order to control the degree of recirculation the volume control damper in the bottom is adjusted. The volume of fresh air entering the station is indicated by the manometer and orifice plate at the fresh air intake point.

The volume of air being passed in the air conditioning process is given by the in duct orifice and associated manometer.

6. Measurements

The following measurements are taken:

Lab barometric pressure, Humidification boiler settings used, Dry-bulb and wet-bulb temperatures at all stations along the air-path. Of particular

interest are the values at sections B and C, Pressure differential across the orifice plate at Station E, and Mass of condensate and time for condensation.

Working sheet

Sl.No. Dry bulb temperature at B

Dry bulb temperature at C

Wet bulb temperature at B

Wet bulb temperature at C

1 to 10 readings

A-before steam injection, B-before cooling coils, C-after cooling coils, D-after heater

7. Calculations

The mass flow rate of the air is taken from the orifice readings. Calculate the air-mass flow rate from the pressure drop across the orifice plate according the formula provided in the manual.

ma=0.0517√ zν E

1

The value of the specific volume can be obtained from the psychrometric chart using the dry bulb temperature and the wet bulb temperature of air.

Locate points B and C on the psychrometric chart and determine the specific humidity ωB ,ωC and the corresponding enthalpy hB and hC .

Since condensation process involves the removal of moisture when the air reaches the dew point temperature, it can be obtained if the difference of the specific humidity of air is known. Apply the mass balance of the moisture content of air between sections B and C to obtain the rate of condensation.

mcondensate=ma (ωB−ωC ) 2

Apply the 1st Law of thermodynamics between sections B and C and estimate the rate of cooling.

Qcooling=ma(hB−hC)−mW hW 3

Estimate the mass rate of condensation, mcondensate , from the mass of condensate divided by time of condensation; compare with value obtained previously.

The cooling rate provided by the coil can also be determined by the enthalpy difference between the evaporator outlet and inlet for the refrigeration system. Calculate the rate of cooling provided by the coil.

Qcooling=mref (h1−h4) 4

where : mref is the refrigerant mass flow rate , h1 , h4 are the refrigerant enthalpies after and before the evaporator coil. Compare with the QCooling determined in (3) previously.

8. Nomenclatureo z Orifice differential head (mm H2O)o vE Specific volume of air calculated by assuming air to be a perfect gas.

νE=RT 9

PLab−P ν≅RT 9

PLab

m3

kg (dry air )5

o ma Air mass flow rate in kg/seco ω Specific humidity, kg/kgo h Specific enthalpyo mref Refrigerant mass flow rate ,o h1 , h4 Refrigerant enthalpies after and before the evaporator coil.

9. Reference

Stoecker W.F., Refrigeration and Air Conditioning , McGraw-Hill Book Company , 1989