MAC 7/8 Reverse Cycle Defrost Issue
Had a call to a McDonalds today. They have a 3 month old MAC 7/8 system with the two digit display and only two buttons on the controller. The freezer evap was VERY frozen up. I put the machine into manual defrost. The compressor cycles on and off on the low pressure switch maybe 10 seconds on, 10 seconds off. The LPC was set to cut-in at 30psig and out at 10psig. Being that it was cold outside the suction pressure in defrost was causing it to cut out. I manually cleared all the ice off the coil with a hose then restarted the unit and let the box get to temp. While this was happening I checked all the settings in the Master Controller and found nothing out of sorts. Once the box was nearing temp (and it did so fairly quickly), I put it back into defrost. In the past I have had Master-Bilt tell me to set the cut in/out on a reverse cycle to about 20-25 and 2-3 for 404A, so I reset the LPC to about these points and the compressor did not cut out. The coil cleared and defrost terminated, though it took nearly 20 minutes.
I am wondering if the charge is suspect? The unit has a sightglass, but I haven't found them terribly useful for these systems since they normally bubble quite a bit. Superheat is maintained during freeze, which makes me think it isn't undercharged. The sightglass bubbles during freeze, so this makes me think its not overcharged. But, defrost takes a long time and the suction pressure at the compressor is riding the edge of cutting out on the LPC during defrost even with my resetting. The only way I know to properly charge these things is weigh in a calculated amount, or a little less, from the installation manual, then add small amounts while watching the superheat on the controller. I didn't install this unit, so the only way to verify charge is to pull it out.
Let's talk about this a little -
When the system is cooling - what is the subcooling? I think you would want it pretty generous in this application. And I can tell you: having both low head pressure (discharge temp) and low suction pressure would sure have me looking for an excuse to add refrigerant. <g>
The outside coil is the evaporator in defrost. So take the lowest likely ambient temperature, subtract 20º from it, and then chart that number for the system refrigerant to get the lowest likely operating suction pressure. Set the LPC to a few lbs. under that.
"evap very frozen" ? How many defrosts per day? With reverse cycle defrost I would like to see four or six short defrosts - maybe 4-5 minutes each. Defrost is to De Frost the evaporator - not to de-ice it.
Originally Posted by MicahWes
a 54º condensing temperature ?
Thanks. That was After you added refrigerant? Any idea what percentage of the total system charge one pound is? In other words; did you add 5% to the charge or did you add 35% to the charge? <g>
Does this system use a receiver? If so; the next time you are there - can you take a picture of it for me?
R-404, right? So how can the system work properly with a 113 lb. head pressure? Isn't the head pressure controlled to a certain minimum in some way? How low is the suction pressure when the system is cycling normally? Isn't it trending in the twenties somewhere? How much minimum pressure drop is required across the TXV to insure accurate refrigerant metering?
What is the head pressure when the system is defrosting?
Originally Posted by MicahWes
if you are holding the head pressure low -
If you are holding the head pressure that low - where does the heat come from when you reverse the cycle to defrost?
Originally Posted by jpsmith1cm