Coffin Case Woes

[Stiff Nipples]

I am having problems with some coffin cases (Hussman model type - GWIT). That keeps buliding frost in the discharge plenum of the case. The case drains well, and the coil is clear, What are some things I can check?

[phil68]

Increase drip time after defrost

[chillin out]

Defrost termination tempature set too high.

No fan delay.

Chillin

[Stiff Nipples]

Thanks for the replies. The drip times have been increased. The fans run all the time so if I were to install a fan delay I would be preventing the fans from pushing that moist air throughout the case. Is that correct? Also if the defrost termination is set to high how does that affect the build up frost?

[chillin out]

Also if the defrost termination is set too high how does that affect the build up frost?

If it is set too high not only will you defrost the coil but you will turn most of the water into vapour. Which will condense on the back wall, drip down and then refreeze at the bottom.


Chillin

[phil68]

As Chillin Out said you also need a fan delay after the defrost so that any water that is left on the coil is re-frozen instead of being blown out by the fan.

[Stiff Nipples]

Thanks again I will take it all into account. Do either of you guys know a good site for literature for theses older style cases. The ones I work on are ussually refurbished
And a bad refurbish (coat of paint and out the door ). Would like to make sure that the cases come like they should have if they were new.

[Dan]

Instruct the store to use night covers, and provide a single defrost for 60 minutes when the store is closed. I am guessing that you have Gas defrost, but the advice applies to electric as well. One defrost. Make it long enough to circulate warm air throughout the case.

[Stiff Nipples]

The store staff are not reliable enough to ensure that the 288 ft of coffin that eventually encounters the problem will get covered. Also would a defrost of that length be good for the integrity of the product?

[Dan]

One good and complete defrost every 24 hours is your best option regardless of the store's discipline or lack thereof. Clearing the flues and drain pan are an important part of the defrost process, thus a longer defrost only once a day is the best option. Product integrity is best preserved by having as few defrosts as possible. Not by shortening the duration and increasing the frequency, which is an all-too-common occurence.

[arkay]

We have good results here with two defrosts: One just after store closes and the other about 1 hr before opening, to ensure the coils are clean in the morning.

[Refrig_Controls]

One way to accomplish this functionality is to replace the older electromechanical controls with one low cost electronic controller. I work for Carel and we have a solution that would allow you to make adjustments to defrost durations and frequency using one controller. The controller is called a PJ32 and can be found here -

www.carel.com

[Gary C NM.]

We had the same problem in several stores. all with (kool gas) defrost. the best cure I found was to install fan delay switches and increase the drip time. Also found the tech's setting the temps too cold. Warmed back up +5 deg. -10F frozen food & -15F IC.
and it seems to have fixed most of the problems.
Note Here in Albuqueerque we have very low humidity, and thin air. May or may not help in a much more humid climate.

[Karl Burke]

Im sorry but I have not been able to identify whether you have electric or gas defrost as this makes a big difference in regards to temrmination temps and where are you getting you termination from i.e pressure switch, coil temp or air temp.. I agree one defrost per day is best.
You said you had 288 ft of case , are these piped as individual branches and if so do they defrost at different times and do you have branch dividers between the relevant cases?

[Dan]

Just dawned on me that you are using GWIT cases which are "Twin" cases. You might want to make the defrosts simultaneous for each side of case. Picture this: One side is circulating moist defrosting air while the other is cold. It is very likely that the insulated partition between the two discharge flues is permitting the moist defrosting air to condense on the common wall because the other side is refrigerating.

This moisture refreezes and rebuilds after each defrost.

If you cannot simultaneously defrost these cases, then I strongly recommend that you have a single, long defrost once a day during off hours.

You absolutely must let the fans run to permit the de-icing of the flues and drain pan.

[Andy T]

We had same problem on electrulux cases.The construction of the case meant the air off evap space was galvanized steal.This would stay very cold and when the moisture from the defrosted coil passed over it.It would freeze to it.We would move the defrost termination stat coil into the evap. And Set it to 7deg term this allowed the full coil to defrost without over heating it.The newer cases came with a heater in the air off space.Also if there is more than one case put a system divider in to stop air migration from one case to the other.4 defrosts a day 7deg termination 20mins max was the standard within are specifications.If we needed to change that we would have to give a good reason.

[Dan]

Andy T, If it works for you, let it work for you. I am not one who suggests fixes for things people are happy with. But I hope you can see how much farther you can spread the duration between defrosts just in case a future complaint arises.

But a happy story is to arive at any time of day in a store and walk with the frozen manager checking the quality of his product, without making an apology to a defrost.

[chillywilly]

typically a case will exit defrost in one of 2 ways. Either the defrost termination (Temp) or by time (back-up for safety). If the defrost termination has failed or is located in the wrong area the system will remian in defrost until it terminates in time. This situation is usually how ice forms in the discharge (the heaters actually cause the water coming off of the coil to steam, which rolls out into the discharge).

[displaced paddy]

Hussmann recommends one defrost a day, 60 mins if electric defrost (finned heaters on air entering side of coil), 26 mins if hot gas + 5 min drip, 35 mins if Koolgas + 5 min drip. NEVER turn off the fans, NEVER cover the top of the cases, ALWAYS defrost both sides of a wide island case at the same time. Good luck to ya looking for mint GWI or GWITs. They haven't made them in 20 yrs, any you find now are usually on 3rd or 4th install

[displaced paddy]

Also I've never seen one with any kind of termination on it, make sure all the fan blades are moving air the right way and not flattened out and lastly all the baffles or honeycombs that direct the airflow must be in place and clean. Those thing are energy hogs, I cant believe anyones still using them.

[lowcool]

my back hurts thinking about em

[displaced paddy]

Ahh yes, Guinness. Its so much more than just a breakfast drink

[coolhibby1875]

Hussmann recommends one defrost a day, 60 mins if electric defrost (finned heaters on air entering side of coil), 26 mins if hot gas + 5 min drip, 35 mins if Koolgas + 5 min drip. NEVER turn off the fans, NEVER cover the top of the cases, ALWAYS defrost both sides of a wide island case at the same time. Good luck to ya looking for mint GWI or GWITs. They haven't made them in 20 yrs, any you find now are usually on 3rd or 4th install

all the drawings i have seen say that fans should stop when term temp has been reached, then they come back on after time has elapsed!

[lowcool]

here here paddy,bottoms up.
gluk gluk gluk ahhh

[displaced paddy]

I think they assume the warm moist air will rise out of the case(very low velocity air curtain) I've never heard of anyone stopping the fans after defrost, why would you? I've worked on hundreds of these and never heard of any fan controls at all. The problem, if the coil is clear, is usually improper airflow from fans,blades or honeycombs, or air curtain disturbance from a/c grilles or door openings

[vadivelan]

I had a simlar problem at one of the site. The basic thing about the fan operation in Freezer display case is as below

Freezer Display with glass doors: Fan Off during defrost.

Open Freezer Display: Fan "on" during defrost to maintain the air curtain.

We have increased the evaporating temperature from -36 deg C to -33 deg C and increased the defrost duration from 20 minutes to 35 monutes. Since the store humidity condtions are not being maintained, we were forced to go for more number of defrosts than recommended. With this arrangement whatever little frost forms on the supply grill during the cooling cycle gets melted during defrost cycle and we could solve the problem.