MAC 7/8 Reverse Cycle Defrost Issue

[MicahWes]

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.

[trippintl0]

I've never worked on a Mac 7/8, but all the other Mac Racks I have worked on are Heatcraft, and I've never heard anybody say the sightglass should have bubbles? All the ones I saw bubbles on, I found leaks on, fixed leaks and recharged to clear sight-glass. These systems had only standard off cycle defrost for the cooler and electric for freezer, so granted it is different from what you're working on. Good luck, and I'm curious to know what you find out, so don't forget to post the solution so we can all learn.

[MicahWes]

Yeah, they are different. No head pressure control, no receiver. I know that if you clear the sight glass during initial pulldown you are WAY overcharged. That EXV can open really wide.

[MicahWes]

When I think about it, it may be just fine. I mean, the condenser is the evaporator when in defrost. It was pretty cold out today, so it stands to reason that the pressure in the condenser (evap) would be pretty low in defrost, and therefore there would be little heat available for the defrosting of the unit cooler coil. I'll check on it tomorrow or the next day.

[Poodle Head Mikey]

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.

PHM
-------

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.

[trippintl0]

Yeah, they are different. No head pressure control, no receiver. I know that if you clear the sight glass during initial pulldown you are WAY overcharged. That EXV can open really wide.

Interesting that there is no receiver. I know the Beacon EXV's don't really require head pressure control as they can operate with very little pressure differential. And before the Beacon's, McDo used the Bohnimizer which also didn't require head pressure control. Cool stuff. (Although I have seen a lot of those older racks with fan cycle controls.)

[MicahWes]

I went back by here today to just check on things. Ended up staying 3 hours. The bottom rows of the unit cooler coil were icing up again. MasterBilt told me to check the charge by checking to see how far the EEV was open during freeze with the box near temp. I found it running between 60-70% open. They say this indicates an undercharge. I added 1 lb of refrigerant and rechecked the valve. It was then operating between 30-40% open, and this is what they want to see. The undercharge really doesn't affect defrost much though. I did a little looking around and found that the defrost termination thermistor was mounted on the lowest distributor tube. At first I thought this was the perfect place, since the bottom rows are the ones that aren't clearing. Then I noticed that the drain pan heater was right beneath it. I theorize that part of my problem is that the pan heater is warming the thermistor and causing early terminations. I looked in my MAC 7/8 manual and in the installation diagrams it does indeed show the thermistor mounted at a distributor tube halfway up the coil. I moved it up, mounted and insulated it. I cleared the coil manually and restarted the box. I hope this might help solve the problem. I also (at MasterBilt's suggestion) moved the max defrost time from 25 minutes to 35 minutes.

[MicahWes]

Oh, and Poodle Head, the head pressure today was 113 psig and the liquid line temp leaving condenser was ambient (39F). So, subcooling was about 16 degrees.

[Poodle Head Mikey]

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?

PHM
------

Oh, and Poodle Head, the head pressure today was 113 psig and the liquid line temp leaving condenser was ambient (39F). So, subcooling was about 16 degrees.

[jpsmith1cm]

EEVs don't require the same pressure drop across as a TEV does.

One of the advantages of them.

With rack systems, I've got to keep my head pressure up to 70 minimum SCT to maintain that pressure drop across the TEVs. If everything went EEV, I could drop my head pressure another 15-20 degrees with no problem.

[Poodle Head Mikey]

If you are holding the head pressure that low - where does the heat come from when you reverse the cycle to defrost?

PHM
-------

EEVs don't require the same pressure drop across as a TEV does.

One of the advantages of them.

With rack systems, I've got to keep my head pressure up to 70 minimum SCT to maintain that pressure drop across the TEVs. If everything went EEV, I could drop my head pressure another 15-20 degrees with no problem.

[MicahWes]

It comes from the outside ambient, mostly. Condenser turns evaporator, like a heat pump. EEV in unit cooler is restriction device for both refrigeration and defrost.

[jpsmith1cm]

Condensing temp is above freezing, so it will melt ice.

Don't need a 100 degree saturation to melt ice.