Professional Member
Lurker here, working on walk-in today @ one of our stores. This particular store is not open 24 hours. Box was temping fine @ closing. When they opened 6 hours later, box temp was 60* according to manager. I was 2 1/2 hours away. When I arrived, box temp was 42*. Condensing unit is a Heatcraft, part # 89022701, R22, lineset relatively short @ 15'. Outdoor temp 88*. First thing I noticed was lack of condensation on suction line. Looked @ site glass. it was either full or empty. Connected gauges, Low side 22* (45psig) head was 106* (214psig) No subcooling. SH @ condenser was 45*. @ this point I decided sight glass was in fact empty. The box has 3 evaporators in it, each of which showing SH between 24 and 26* SH. Pressure @ each evaporator was also 45psig (22*) I had to damn near empty an entire 30lbs cylinder to fill my sight glass, give me 10* subcooling (head now @ 225psig, still seemed low to me with 88* ambient, although I'm probably wrong) and bring my SH (@ condenser) to 20*. SH @ each evap ranged from 12-14*. I observed no indication of a leak anywhere in the system. After an hour of run time, the box temp only dropped 2 degrees (to 40*, I was also working on a southern store fixtures display case, whole other story altogether) Anyway, sorry for the lengthy post. Anyone have any ideas as to why this box temp is so damned reluctant to fall into the 36-38* range?
Professional Member
I'm thinking, dare I say it, TXV adjustment is necessary. Going through the temping log books, looks like this thing never got below 40* since installation. Trying to figure out how it got to 60*. Defrost timer is set @ 30min once every 24 hours @ 4am. Starting to think the 60* box temp they reported was inaccurate.
Professional Member*
what is the temp control set at ?
Professional Member
Sorry, the unit never satisfied. It was set @ 38*
Professional Member*
You're going to have to be careful here. If you have a empty sight glass the unit is going to be cycling on low pressure control and not holding at the normal 45psig. Also if your glass is empty you will have no cooling and the box wouldn't have pulled down from 60* to 42*. A quick check to find out if it's full or empty is to close the king valve on the reciever and watch the glass.
On a refrigeratin system like this we don't worry about subcooling. To get a lot of subcooling you would have to fill up the reciever and back refrigerant up into the condenser.
I would say the sight glass was clear and the system is now overcharged. Where I live I've found at this time of year the systems that are not adjusted correctly lose control in the heat. As the temperature goes up condenser capacity decreases. As they collect dirt in the summer capacity decreases. High superheat capacity decreases. Warmer in the store more heat load on the box. All adds up to a cooler or freezer that isn't quite cold enough. It's critical to have a clean condenser, clear sight glass, and low superheat at the coil outlet when the box is at temp.
Professional Member*
Also check to see if unit is cycling DF on the termination stat or the safety, in defrost.
AOP Committee/Professional Member*
Beware of advice on the internet.
Your doing good so far. Did you find a leak anywhere, or do you think the system was undercharged?im assuming that this is an r-22 system. After you get 10-12 degrees of subcooling check your superheat at the evap outlet. On a 22 system I would look for 8-11 degrees, some may disagree and want 5 to 9. After you adjust one valve, check your subcooling and give the txv time to settle down. Before going on. Make sure you re-check your superheats and subcooling before leaving.
Professional Member*
One easy way to check sight glass if unsure is just pump it down.
Not what it use to be
Professional Member*
This is kind of interesting to me. I've never known a refrigeration system with only vapor feeding the evaps to be at 42* when I arrived. So without confirming a empty sight glass or a shortage of refrigerant he added 30lbs of r-22 to get his subcooling right? Not a good idea. On a system like this don't concern yourself with subcooling. It is what it is. The 8-11 degree evap superheat is a good target.Originally Posted by Joe Harper
Like itsamine and I said pump it down and confirm the status of the glass before doing anything else. Could have just needed a txv adjustment like I said.
Professional Member
A pump down would have confirmed the state of the sight glass. Good tip. Should have thought of that myself. I returned to the unit today. It was temping @ 36*. I'm wondering if the defrost cycle will have to be lengthened though since the box is running cooler. I guess the crux of it is: Why is subcooling regarded as a valid indication of system charge on an AC system with a TXV but not on a walk-in cooler with a TXV?
AOP Committee/Professional Member*
Subcooling reading is the way to check the charge on a cooler or freezer with a txv.
I am not part of this "we" group that doesn't worry about subcooling...
Professional Member*
Because a refrigeration system has a reciever. The amount of subcooling comes from the design of the condenser. We charge by a clear sight glass. As you add gas your raising the level in the reciver. On a A/C with a txv/fixed metering device or a captube system refrigerant sits in the condenser. The more you have stacked sitting in there the more subcooling you get.
When "we" refrigeration guys work on a system with a reciever we clear the glass then add additional refrigerant for condenser flooding in the winter. Go ahead and check subcooling but don't charge the system to try and achieve a certain amount.
Here's an interensting thread about subcooling and recievers you may enjoy... http://hvac-talk.com/vbb/showthread.php?threadid=56495
Regular Guest
have you checked your TXV screens? i'd start there if you don't have liquid cores.
Professional Member
Well, then I can throw this complicated thing away
http://www.sporlanonline.com/90-30-1.pdf
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