problems with freezer defrosting

Hi,
Is there anybody out there with much experience using strip heaters with ammonia evaporators to help out with hot gas defrosts?

I have three freezers that do pretty well with hot gas defrosting, but run ito problems every few weeks. Even with daily hot gas (75PSI) periods of 45 minutes, there always seems to be a tiny bit that clings to the bottom of the coil. After a few weeks, a bridge will form at the bottom between the coil and the drain pan and the ice will begin to creep up and I'll have to spend some time with it.

I've been wondering about trying some strip heater to keep the bridge from forming, but I also wonder if this solution might bring its own set of new problems.

I know the best solution is to minimize outside air infiltration, but with heavy traffic and a marine climate, what can you do?


Jim Heffernan
Tillamook Oregon
:)

Hi, Jim. The joys of coldstore refrigeration. I too have coolers on various sites that need cleared on a regular basis. On one site we are going to try dehumidification in the loading bay area, this will hopefully cure the ice build on this site. Rod heaters, switched on with the hot gas will also work, but you will have increased pulldown times after defrost. Possibly check the temperature of the hot gas reaching the valve station, maybe it is on the end of a run or to many other coolers are defrosting at the same time. Also I personaly have less defrosts, but for longer times, this cuts down on your pulldown time, and also it means you have a good temperature in the coil area.
Good luck with your coolers, you are not on your own.
Regards. Andy.

Being that the buildup is at the bottom, you might try strip pan heaters.

Jim,

We have had a similar problem with ammonia evaporators with a full 2" strip of ice at the bottom of the coil. In our case it was caused by poor installation where the return pipe from the bottom of the coil had a concentric fitting which meant liquid could log in the bottom pass of the coil and thus prevent correct defrosting. I would check if that lowest pass is getting warm enough during defrost with a surface probe before making modifications.

Simon.

:)

Hi,
Nice to know I'm not alone. Anything that can keep me from spending time on a scissors-lift in a -20F. room seems worth a try. Each evaporator has three fan sections so I'll think its worth a try to do one.

I wonder if way we're doing temperature control exagerates the problem. Except for defrost, there's no pressure control. When we're making ice cream, we have to run the boosters at 13" to get what we want from a spiral chiller. When we're not making ice cream, we shut down 200hp of boosters and pressure goes to wherever 40hp can take it. Pressure floats between 5" and 2psig. All the rooms are flooded units. When temperature gets to set-point, we shut liquid off and turn it back on when temperature hits set-point plus 3. I have to think this leads to a coil that is often only filled 1/2 or 1/4 full.

I keep thinking I would be better off with a coil that was totally full, but running at an elevated pressure. Whenever I bring the subject up, I'm told the pressure drop for a back-pressure regulator is too much for low pressure systems.

Temperature control for rooms work well, so I'm a little in the "don't fix it if it isn't broken" camp. Does anybody out there use back pressure regulators for low temp systems?

Jim Heffernan
Tillamook Oregon

gratefull, yet resentful that his day job keeps him so busy.

Jeff,
I have three -10F evaps cooling a holding freezer that use BPR's. Of course, they are connected to a -40F suction header so there is a good pressure differential across the BPR's.

I agree with Andy that a check of hot gas temps is needed as well as a check of the hot gas line capacity. If the HG supply line is undersized it will take longer to heat the coil enough to fully defrost the coil. If the HG line is extremely undersized, the flow may be so small that the ambient in the room never allows for complete heating of the coil surface.

Perhaps you might look at installing something like a Hansen Frost Master defrost controller that has the capability to accept a defrost terminate input. If the budget does not allow for the Frost Master, I have installed an open-on-rise Tstat remote bulb , set at 35-40F, on the suction line of the valve station before the defrost relief branch or on the bottom of one of the coil fins and wired it in series with the HG solenoid. I then extended the Hot Gas time on the defrost timer and let the Tstat control the Hot Gas time. Took a little time to figure out how long to set the Hot Gas time to cover all seasons and moisture loads. Not a perfect solution as the unit will not return to cooling mode until the timer completes its cycle. But it does help insure that the entire coil is warmed enough to defrost completely without letting the hot gas run for two hours{:<}.

On alot of our freezers we use a gas powered suction check valve and then have a dual pressure bypass regulator to maintain offcycle and defrost temperatures.
Also about having ice left on the bottom of your coil when you come out of defrost how long is your pumpdown cycle before defrost I tend to set mine for 10-15 minutes and run a 25 minute defrost and havent run into any problems.

Hi Industrial Tech,
the coolers I was having problem with were on reverse cycle defrost systems, no valves to go wrong or to play with when things do go wrong:(
I am inclined to believe in my case the problem was the cooler design, we have also used a different cooler manufacturer, and had no problems.:D
Regards. Andy

The problem is most likely insufficient hot gas capacity. (Temperature of the gas has little to do with defrosting.) This is most likely not defrosting due to oil logging in the bottom of the coil. Blow that sucker out!