Subject: Refrigeration pressures - help required

From: John Cain <[email-masked]>
Date: Thu, 03 Aug 1995 11:28:57 GMT

First let me say that I am delighted to post to this group as the 
standard of the threads in general is, in my opinion, very good.

I recently designed some air conditioning systems for densley occupied 
meeting halls in the UK.  I used a 7-Ton heat pump delivering 1.22 m3/s 
supply air of which 0.7 m3/s fresh-air was introduced. 

To maintain a neutral pressure in the space, rather than to waste 
the energy in the return air, I ducted the room extract air (again 0.7 
m3/s) to the inlet of the condenser to mix with outside air (its an 
indoor packaged unit) this moisture laden air cools the condenser air 
in the summer and heats it in winter, improving overall efficiency.  
Has anyone any comments with regard to this modification?

I visited one system yesterday which had tripped-out on head pressure.  
When I connected my gauges I recorded a head pressure rising of 375 psi 
and suction of 75 psi (external air temp: 30C, internal room temp 25C). 
The condenser was clean and no leaks were evident.  I noticed that the 
extract duct to the condenser had been removed, when this was 
reinstated, the head reduced to 325 psi.  The unit was correctly 
charged with 6.76 kg of R22 (manufacturers recommendation).

I wonder, what affect higher head pressures have on the efficiency of a 
system?  As I see it, the result of increasing the condensing 
temperature is a decrease in the COP due to the increase in compressor 
power.       

In my opinion the condenser is not removing sufficient heat to reduce 
the head to say 250-300 psi (I would be much happier with these 
pressures) which would reduce suction pressure to say 60 psi and hence 
provide a cooler evaporator.  I know I should have measured the actual 
cooling capacity across the evaporator but wonder what percentage 
reduction in capacity I could expect with these higher pressures?  I 
accept that sufficient detail has not been provided for an in-depth 
reply but comments would be appreciated.

Many thanks.   

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| John Cain I.Eng ACIBSE MASHRAE    EMail: [email-masked]   |
| BSC Building Services Ltd         Tel:   01908 231128  Fax: 231129 |
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