Why the evap coil ices up - DIY Chatroom

thehvacguy · 10-30-2009, 05:42 PM

I just can't understand why the evaporator coil would ice up if the system is low on refrigerant. I would think that since the evaporator is starved and the refrigerant is boiling off quickly there would be no reason for it to be cold enough to ice up. Can anyone offer clarification on this subject?

yuri · 10-30-2009, 05:50 PM

Look at a pressure temp chart or the one on your gauges. For an R22 system if it is running say 35 psig on the suction side that corresponds to a 12 deg F coil which is 20 degrees below freezing. The coil freezes the water/condensation. In order to have a 32 deg coil you need to run at 57 psig. It is not quite that simple with total load and amount of airflow over the coil but the basic fundamentals are pressure=temperature.

thehvacguy · 10-30-2009, 06:12 PM

Oh, thats right. Lower pressure=Lower temperature. Off the top of my head I know that r22 boils at 40 degrees, which corresponds to 70psig (or something like that). So actually if you are overcharged the unit would not remove as much heat?

yuri · 10-30-2009, 07:10 PM

Its a LOT more complicated than that. Have you taken a refrigeration course at a trade school?

thehvacguy · 10-30-2009, 07:15 PM

yes I am actually taking a Residential HVAC Course at Wyotech in Long Beach CA and I came across this question during class and I cant ask my teacher cause he's an ass. I know that there is a lot that can go wrong in the refrigeration cycle. The evaporator coil could be dirty restricting the airflow, which would cause the coil to ice up. But I was on youtube looking at videos and there is a guy that puts HVAC vids on there named Dr Zarkloff who stated that another reason that the coil will ice up is an under charge and I didn't quite understand that.

yuri · 10-30-2009, 07:25 PM

Be careful with some of those videos and guys as some of them are pretenders/wannabees and joke videos. Having said that I would recommend you THOROUGHLY learn the fundamentals of refrigeration. Those being pressure/temp and saturated refrigerant vapor/superheat/subcooling etc. I am a Journeyman and we have several WELL educated Dudes here but you need to understand those basics before we can help you. Psychrometerics and latent heat and the basic chemistry stuff is VERY important later. There is a great textbook called the Principles of Refrigeration, by Dossat. A lot of courses use it, sort of a bible for the industry. Expensive but worth it.
http://www.amazon.ca/Principles-Refr...ion/0130272701

thehvacguy · 10-30-2009, 07:31 PM

I am in my third month at Wyotech and this month we are learning the basic refrigeration cycle. I am kind of understanding this stuff. I mean, I have been in the trade for about four years, and my dad worked for Johnstone supply since I was born. So I have been exposed to the biz my whole life. its just that the technical stuff takes a couple of times of explaining for me to really understand why it does what it does.

yuri · 10-30-2009, 07:35 PM

Check my previous post again for more info. You REALLY REALLY need to understand the basics as 95% of units in the future will use thermostatic expansion valves and the air handlers will have ECM variable speed fans and that affects EVERYTHING a lot.

beenthere · 10-31-2009, 12:06 AM

Did you see his video with him(Dr Zarkloff) wearing his shoes on his head.
Aliens told him to do this.

So watch what you believe from him. Take it with a grain of salt.

But, yes, the evap coil will freeze over if the system is too ow on charge.

If your instructor is a (whatever you want to call him) how are you going to learn anything if you can't ask him?

What text books are you using?

The refrigeration process is easy, and hard to understand.

thehvacguy · 10-31-2009, 02:47 AM

We are using the NCCER Third edition. Well, pretty much we go into the lab every day and learn it ourself. The teacher demonstrates and we follow along. Today we learned how to evacuate a split system. We watched him do it and then we did it three or four times.

beenthere · 10-31-2009, 06:21 AM

Should have a lot of theory also.
Haven't seen or heard too much about NCCER HVAC modules.

Hopefully, he also goes into what to do on systems that don't want to pull below 1000 microns. That don't have leaks.

And if your real lucky. He'll teach you why your guage set/hoses is not a good tool for pulling proper vacuum.

thehvacguy · 10-31-2009, 01:49 PM

Well I know that the gauges are not good for pulling a vacuum because you may think you are done but there are still non condenseables in the line. A micron gauge is more accurate. Why would you not be able to pull a vacuum lower that 1000 microns? The reasons I can think of is if there is a leak, or mabe the vacuum pump is too big for the system and the moisture in the line will freeze before it can be removed

qbert · 10-31-2009, 04:19 PM

Use vac hoses or copper for better results

thehvacguy · 10-31-2009, 05:03 PM

I didnt understand your last post. I use vacuum hoses when I pull a vacuum. I have never seen anyone use copper to pull a vacuum. Mabe do you have a pic?

beenthere · 10-31-2009, 05:33 PM

When techs use the manifold sets to pull a vacuum. The hoses are often contaiminated from the systems they were previously working on. And the comtaminates continue to off gas while they try to pull a vacuum.

What are you calling a vacuum hose?