Prepare to Mobilize: 10 Steps to Prepare Your Business to Go Mobile
Oct. 28, 12 p.m. EDT – Learn how to get the most out of your mobile investment - Click here for details

Air Conditioners / Refrigeration

The Professor: The Needed Amount of Condenser Subcooling

April 2, 2012
/ Print / Reprints /
/ Text Size+

[Editor’s Note: This is the third in a three-part series on the importance of liquid subcooling in a refrigeration or air conditioning system.]

The amount of condenser subcooling needed is system dependent. The more pressure drops — friction and static — associated with the lines and accessories that carry the liquid in the system, the more need there will be for liquid subcooling to prevent liquid line flash gas. A quick review of the second article in this series (“Why Is Condenser Subcooling Needed?” published in the March 5, 2012, edition of The NEWS) will remind readers of the differences between friction and static pressure drops.

The only sure way to safeguard against liquid line flash gas is to subcool the liquid before it experiences any pressure drops.

Subcooling can be accomplished by numerous methods. Liquid/suction line heat exchangers, ambient air, and direct expansion mechanical subcooler heat exchangers are common methods. On receiverless systems, simply adding a little more refrigerant to the system can increase subcooling but usually isn’t recommended because of added inefficiencies that may occur.

Now, let’s figure out what the minimal amount of liquid subcooling in the condenser would have to be in order to prevent liquid line flash gas on the way to the metering device.

Assume the following conditions:

• Filter drier pressure drop is 1 psig (friction pressure loss).

• Design pressure drop for friction losses in the liquid line is 2 psig for copper pipes.

• Vertical lift of liquid is 40 feet. This equates to a static pressure loss of 20 psig (see Figure 1).

• The system has a condensing pressure of 211 psig (105 degrees F).

Solution: The total pressure drop for this system is 23 psig (2 psig + 1 psig + 20 psig). The 23-psig pressure drop puts the new pressure at 188 psig (211 psig – 23 psig). The saturation temperature that corresponds to 188 psig is about 97 degrees (see Figure 2). Therefore, the liquid must be subcooled to at least 97 degrees to prevent liquid line flash gas. This means the system would need at least 8 degrees (105 degrees – 97 degrees) of condenser subcooling to prevent liquid line flashing before getting the metering device.

If the pressure drop isn’t exactly known, make sure there is enough subcooling to handle an extreme pressure drop. However, on receiverless systems, be sure you do not overcharge the system by adding too much refrigerant. Overcharging will increase the condenser subcooling, but the head pressure will now elevate and give unwanted inefficiencies from higher compression ratios. Always consult the system manufacturer for the proper amount of condenser subcooling if charging to this specification.

Publication date: 04/02/2012

Did you enjoy this article? Click here to subscribe to The NEWS Magazine

Recent Articles by John Tomczyk

You must login or register in order to post a comment.



Image Galleries

2014 SMACNA Annual Convention

Views from the 2014 SMACNA Annual Convention in San Antonio.


NEWSMakers: Chris Mathis

Chris Mathis, president of Mathis Consulting Co. and vice chair of ASHRAE’s residential ad hoc committee, talks about ASHRAE’s exploration into the residential market. Posted on Oct. 17.

More Podcasts


NEWS 10-20-14 cover

2014 October 20

Check out the weekly edition of The NEWS today!

Table Of Contents Subscribe


DOE has denied AHRI’s petition for reconsideration of walk-in standards. Is it getting harder to reach consensus on efficiency standards?
View Results Poll Archive


2015 National Plumbing & HVAC Estimator

Every plumbing and HVAC estimator can use the cost estimates in this practical manual!

More Products

Clear Seas Research


Clear Seas ResearchWith access to over one million professionals and more than 60 industry-specific publications, Clear Seas Research offers relevant insights from those who know your industry best. Let us customize a market research solution that exceeds your marketing goals.


Magazine image
Register today for complete access to Get full access to the latest features, Extra Edition, and more.


facebook icontwitter iconyoutube iconLinkedIn i con