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07-14-2008, 08:53 PM #1Regular Guest
- Join Date
- Nov 2004
Freezer box temperature rise during OFF cycle
I have a problem with the temperature rise during compressor OFF cycle in a 20cu ft reach-in freezer. The system is R404A, capillary tube, heated glass door. The freezer reaches -20F, then the electronic controller turns off compressor, evaporator fan stays on. The controller differential is 10F, but the controller has a compressor minimum rest time 6 minutes. During those 6 minutes, the box temperature rises to about -3F to -2F. This temperature rise is excessive. I would like to see the box temperature not exceed -8F during the off cycle. The low side pressure is 9PSIG to 10PSIG, high side is around 240PSIG at -15F box temperature and 78F to 80F ambient. It looks that the greatest factor in this temperature rise is the warm refrigerant moving from the condenser to the evaporator. This refrigerant migration causes the evaporator coil to warm up, then the evaporator fan transfers the heat to the box. I was experimenting with the fan to have it cycle with the compressor, and yes the rise is slower during the off time, but, when the compressor and fan start, the box temperature jumps suddenly to about the same -2F or even slightly higher. I would like to avoid this option of cycling the evaporator fan for various reasons.
Has anyone experienced a similar problem - is there any way to correct this?
What temperature rise do you guys see on glass-door, reach-in freezers of this size?
07-14-2008, 09:06 PM #2
The displayed temp sensor is most likely in the air stream or the evaporator. Some units have several sensors, and during defrost you do not want to display the evap temp, so they will switch the displayed sensor input. -20 is pretty low, and unlikely to be found on a standard r\i freezer as the holding temp. That is closer to the evap temp. There will be a very low twmp rise if the defrost is set correctly."Fighting Ignorance since 1973 (Its taking longer than we thought)." The Straight Dope.
07-14-2008, 09:30 PM #3
did you ever try to shorten your differential and see what happens?
07-14-2008, 11:49 PM #4
07-15-2008, 09:39 AM #5
Worried about a temp. rise in the off cycle? If you solve that problem. What do you expect to happen in the defrost cycle?
07-15-2008, 10:17 AM #6
On another thought, Wire the condensor fan motor direct so it doesn't stop when the compressors shuts off. Should help cool the high side refrigerant to ambient temp a little more quickly.
07-15-2008, 11:33 AM #7Professional Member
- Join Date
- Nov 2007
How much product is in this freezer? Could have alot to do with how fast air temp. is rising, product should help maintain the box temp. once it is cooled to your setpoint. If you are reaching a cut in temp. which you say is 10* differential and case is pulling to -20* before six minutes you have somthing generating alot of heat into this case. In my opinion the liquid migration issue from condensor should not warm the case at this rate.
07-15-2008, 06:28 PM #8
I signed on to this......
Liquid migration causing box temp to rise more than 15 degrees in less than six minutes? Wiring condenser fan motor to run all the time, to reduce the liquid line temp? Put a liquid line solenoid in a freezer designed for equalization? What in the heck are we doing here? Decreasing the differential to see what happens?
This unit has a controller to operate the unit (noted by compressor protection of six minutes). The controller LIKELY has input sensors located in the evaporator AND in the box. It is LIKELY switching the display between the sensors when cycled off or in defrost. If you decrease the differential, it will still do the same thing if the temp is actually rising this much within six minutes. Liquid migration on this unit is insignificant, and not relevant.
What is the manufacturer and model number? This makes a big difference, as all units are not the same...... My guess it is a Delfield.
Last edited by ckone180; 07-15-2008 at 06:37 PM."Fighting Ignorance since 1973 (Its taking longer than we thought)." The Straight Dope.
07-15-2008, 07:58 PM #9
You mean this isn't a joke ?
"warm liquid migration to the evaporator" ? <g>
Come on now - that process is called: refrigeration. <g>
Hint: it's not warm after it gets there. <g>PHM
07-15-2008, 09:40 PM #10Regular Guest
- Join Date
- Nov 2004
The temperature rise was measured with empty box - no products. I was monitoring temperature by two means: freezer electronic control and temperature logger with thermocouple tip at the evaporator fan guard. The electronic control probe is mounted on the inner top, between the inner side and the evaporator shroud, about 5" away from the back wall. Both the electronic control and temperature logger are showing approximately the same rise.
Nothing unusual as far as heat load: two fluorescent lights on sides, 9 watt evaporator fan, triple-pane glass heated with approx. 40 watt, 2.25" thick walls.
It is definitely warm refrigerant migrating from the condenser that has the largest impact. The moment compressor is turned off the pressures are 10PSIG low side, 240PSIG high side, then the pressures slowly equalize - reducing high side and increasing low side. When the compressor is about to start, the pressures are equalized at approx. 32 PSIG. That is equivalent to 0F saturated suction temperature. In my opinion, the air moved over approx. 0F evaporator coil is a major contributor to the temperature rise.
This temperature rise does not spoil the products, but confuses the customer.
I have to see how much it goes up with the products.
Having a solenoid valve at the capillary inlet, wired in parallel with the compressor and the fan, will stop the refrigerant migration. My concern is the impact on the compressor start-up conditions - will the compressor handle the large unbalanced pressures in long term, will the circuit breaker blow?
07-15-2008, 09:59 PM #11
07-15-2008, 10:08 PM #12
My experience is empty box does not hold cold well
07-15-2008, 10:52 PM #13
ckone 180 i jumped in without thinking . i know after reading the whole post that the differential has no play in the question asked. sorry guys