When Subcooling is Meaningless - Short #161

HVAC School - For Techs, By Techs

Bryan Orr

Training, Careers, Airconditioning, Self-improvement, Hvac, Business, Education, Refrigeration, Heating, Ac, Apprenticeship

4.8 • 985 Ratings

🗓️ 25 April 2023

⏱️ 10 minutes

🔗️ Recording | iTunes | RSS

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Summary

In this short podcast, Bryan talks about the times when subcooling is meaningless. It is important to understand subcooling fully before using it as a charging or diagnostic method, particularly in refrigeration systems.

When we need to charge TXV systems in residential HVAC, many technicians rely on subcooling to set the charge. However, refrigeration systems don't quite work the same way; charging a refrigeration system by subcooling may lead you to overcharge the system.

We take subcooling on the liquid line between the condenser and the metering device. The condenser takes superheated vapor and rejects heat. This process turns the superheated vapor refrigerant into a liquid-vapor mixture midway through and subcools the liquid refrigerant at the bottom of the condenser. That liquid "stacks" at the bottom of the condenser. Adding more refrigerant will cause more liquid refrigerant to stack up at the end of the condenser and increase subcooling. These conditions can cause an increase in head pressure.

However, many refrigeration systems have receivers between the metering device and the condenser. Excess refrigerant gets stored in the receiver; it doesn't stack up in the condenser, and it doesn't contribute to additional subcooling. As the liquid line fills and the metering device restricts, the liquid goes into the receiver, not the condenser. Subcooling won't change much until the receiver is full, which is a major problem; receivers should only be up to 80% full, even when pumped down. So, we rely on sight glasses or receiver-level monitors to determine the charge, not subcooling. The goal is to keep the sight glass clear, meaning there is a full line of liquid going to the metering device. (However, sight glasses will also be clear when the system is off or empty.)

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Transcript

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0:00.0	Welcome, butting HVAC super tax, or those of you who have made it.
0:08.0	Those of you who have been doing this 20 years and never had a problem.
0:13.0	This is the HVAC school podcast, the podcast for Tex, by Tex, I'm Brian, and today we're going to talk about
0:19.8	sub cooling and why it's important to understand it fully before using it as a diagnostic or
0:26.2	charging method. Again, subcoolings used a lot for a charging method and a lot of you may say,
0:30.6	well that's how you charge a system in fact if you ask a lot of
0:33.0	techs what sub coolly means they'll say it's how you charge a system with the
0:37.2	TXV but nay I have a caveat and that is for the refrigeration technician or more specifically for the
0:45.1	H.E. technician who finds himself working on refrigeration as we oft do in
0:49.5	smaller environments like some restaurants or convenience stores, things like that where you're working on the AC and then you get asked to look at the refrigeration and
0:57.3	You got to understand some of these things before you think that you can just use subcooling so, this in all of our episodes are targeted at professionals.
1:04.8	And so a lot of people say, if you don't know that,
1:06.6	you should go back to school.
1:07.6	But again, we know that as technicians,
1:09.8	and many of us junior technicians,
1:11.3	maybe only done it a few years, you get thrown into situations
1:14.2	where you enter of your head and you are tasked with trying to figure it out.
1:16.7	So this is a podcast, a short podcast that's here to help you with that.
1:19.8	But before we do that, let's thank our great sponsors. Navac at Navac Global.com
1:26.4	Refrigeration Technologies at refriggedec.com.
1:30.3	carrier and carrier.com I've been a carrier dealer for many years.
1:34.8	Carrier has their new green speed extreme super high efficiency heat pumps out on the
	...

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