Objective
To investigate wet surface cooling and dehumidification of air.
Method
By humidifying air using the introduction of steam to the air stream. By cooling this humidified air using a compression-based refrigeration (cooling) unit. By performing mass and energy balances on the system. By varying the moisture content of the air to investigate the effect this has on the results obtained.
Equipment Required
RA2 Air Conditioning Unit
Compatible PC (not supplied by Armfield) RA2-304 Software
Optional Equipment
Barometer for measuring local ambient pressure (if not available then some alternative is required, such as a local weather report or an appropriate default value).
Theory
Cooling with dehumidification can be represented in diagram form as follows:
hB hC
wB wC
maB maC
mcond
h4 h1 hcond
Mass and energy balances may be performed as described in earlier experiments. The electrical power input into the compressor is logged by the software. This is taken to be approximately equal to the energy consumed by the compressor in doing work.
Mass balance
From continuity equation:
a aB
aC m m
m& = & = &
From this, the mass flow rate of steam can be calculated as: ) w w ( m w m
m& cond = &a ⋅∆ = &a B− C
Hence total mass flow of moist air [kg/s] is: ) w 1 ( m m
m& a − &cond = & a −∆
Energy balance
comp
BC
H
W
Q&
=∆&
−
&
∆
comp
W& = Compressor work ≈ Compressor power input
[
a B C cond cond]
comp.
BC m (h h ) m h W
Q = & − − & − & ∆
Air enthalpy change can be calculated as: )
h h (
m&a B − C
Condensate enthalpy can be calculated as:
cond condh
m& (assume hcond = 419.04kJ/kg )
Equipment Set Up
The boiler is required for this exercise, and should be filled before use to MAX LEVEL (as indicated on the sight glass) with clean, preferably distilled or de-ionised (demineralised), water. The filling procedure is described in the Operational Procedures section of this manual (see 3.3).
Ensure that the equipment and PC have been set up as described in the installation guide, and that the PC is connected and switched on with the RA2-304 software running. The software should indicate ‘IFD: OK’ in the bottom right of the software
Procedure
NOTE: Where the ambient temperature of the laboratory is low, better results may be obtained with the use of the preheater. Exact requirements will vary as the ambient humidity will also affect the results, and so are left to the discretion of the individual user.
In the event of extremely high ambient humidity (relative humidity close to 100%), it may not be possible to investigate cooling over a sufficiently wide range of humidity values, and in such cases it may be preferable to postpone the experiment until ambient conditions are more favourable. However, in high ambient humidity conditions it should be possible to investigate dehumidification without needing to humidify first (i.e. without using the boiler).
In the software, set the fan speed to 40% and turn the cooler on.
In low to moderate humidity conditions it will be necessary to use the boiler to humidify the air. Set the boiler to full power until steam appears at the exit to the steam lance, then switch off full power and set the boiler Manual Output to 25%. Allow the system to stabilise (approximately 25 minutes), then perform a condensate collection by collecting the condensate from the compressor condensate outlet tube over a timed period of 20 minutes. Record the volume collected and the time.
Select the icon and collect data for 15 minutes every 10 seconds. Repeat for a boiler setting of 50% and 75%.
If time permits, the experiment may be repeated at a lower air flow rate by adjusting the fan speed setting. Ensure that the flow rate is never less than 82 m³/hr.
Switch off the boiler (set Mode of Operation to Off). Switch off the preheater, if used.
Allow a few minutes for the temperatures to stabilise, and then switch off the fan. If draining the boiler after use, remember to first allow sufficient time for the water to cool.
Results
The software logs the output from the flow, temperature and humidity sensors. Calculate the mass flow rate of dry air from the flow velocity.
Calculate the mass flow rate of steam from the continuity equation.
Calculate the mass flow rate of moist air by performing a mass balance calculation. Calculate the vapour condensation rate from the times condensate collection values. The software calculates the heat transfer. Check the calculation for one set of sensor readings.
For each set of data in turn, enter the values of temperature and humidity on the h-x diagram and identify the change of state (dew point).
For each set of data, determine the enthalpy, h, and relative humidity, ϕ, from the diagram.
Determine the moisture content drop, ∆w.
Compare the results obtained at different boiler power settings, including the first set of data.
If the experiment was performed at different flow rates then compare the results obtained from this, including the first set of data.
Discussion
What was the effect on the relative humidity and humidity ratio? What was the effect of water vapour content on air cooling, as shown by the experiment? What is the significance of the results when considering air conditioning system design? Discuss the positioning of the humidifier within a system when cooling is to be employed: what would be the effect on the energy balance and efficiency if the air was humidified after cooling and not before as in the RA2?
Discuss methods for improving the accuracy of results. As project work, students may devise a method of determining compressor efficiency.