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kengineering
02-02-2006, 01:11 AM
I have a gravity deli with a finned coil on top and a serpantine tube coil under the stainless deck pan. The coil never quite defrosts completely so it eventually ices up. It is set up for 34*f product compressor cycle controlled by a t-stat. Off cycle defrost, 134A, Timer set for a 20minute defrost 4-times per day. I have been advised that gravity cases should be defrosted once per day for 90-minutes. What do you guys think? Ken

US Iceman
02-02-2006, 01:58 AM
The gravity coils will take longer to defrost, since the air movement is dependent on the natural convection current. The air circulates much slower, so the defrost time can be longer.

On fan-coil units with off cycle defrost (what we could call air defrost ), the fan continues to operate so the defrost time is much faster.

I think the 90 minutes off is too long, unless the defrost occurs when the unit is not being used, say after hours.

Some of the other members may have a better feel for this.

bangoman
02-02-2006, 04:30 PM
I prefer to control icing up using lp switch and thermometer check the low side pressure at which you achieve your desired temp and that will be your cut out on lp switch.Then check the low side pressure at which your coil is clear of ice and has the desired temp and set it to be your cut in on lp switch.

Peter_1
02-02-2006, 06:22 PM
With a gravity coil, we defrost once/day starting with 60 minutes and preferable at night.

If you do it at night, the client doesn't see it, there's no panic that it doesn't cool for an hour and the heat load is then less.

You can also use an evaporator thermostat with the bulb between the fins and set the start point at 2C. It defrost then after each cycle.
Same system they use in many domestic fridges.

Problem you have now is the following: put once an ice bloc and put it in your fridge at 34F and try it to melt.
The moment it start smelting, you put it back in a fridge 24F and makes that the mass grows during the following 6 hour period (time between the defrosts)
Remelt it then again. You know what I'm trying to say: the soon it starts melting, you put it back in freezing conditions and the ice even grows. It will never defrost.

We make many special counters and all the statics are defrosting during the night, sometimes twice.

chillin out
02-02-2006, 07:59 PM
I agree with Bangman, use a lp switch to control the temp.
Those type of coils are not auto defrost, they should be manualy defrosted.

Puting a defrost timer on the case will cause the product to not keep the correct temp and will go off quicker.


Chillin:) :)

Dan
03-02-2006, 12:40 AM
Bangman's method will work fairly well, but the temperature fluctuations may be viewed as undesirable by the time you find the proper year 'round settings. An alternative that is actually some manufacturer's recommendation is to give the case a 120 minute - not 90 - defrost at midnight and let it cycle on a thermostat to control temperature and humidity throughout the day. In my view, this will provide the longest product life. But if you have an ice display, you may find the serpentine coil slowly building up an ice block.

Hill Phoenix makes a case that uses pumped glycol both through the top gravity coil and through the bottom display pans. These cases seem to provide the highest humidity at 29 - 34 deg F product temperatures, which is their selling point. They are currently recommending that you defrost the upper coil once an hour for 5 to 10 minutes... supposedly this helps re-introduce the humidity into the air.

So go figure. :) But I can say this for sure: If you are icing up your case, you should reduce the frequency of defrost and increase the duration.

Yep. I can stand by that. Otherwise, everybody else is correct in their recommendations. :)

Temprite
22-04-2006, 07:06 AM
G'day all

Sounds similar to a problem that I am currently having with a deli display fridge.

In the display section the fridge has a coil at the back, (not overhead) and has another coil in the storage section underneath. System is controlled by an LP switch with an EPR connected to the coil doing the storage section. All are static coils.All superheats are correct on both coils

There is a defrost timer which is giving 3 defrosts a day each with a duration of 30 minutes.I was going to make it 4 times a day but I thought this was a bit much.

Problem is this coils freeze up about once every few weeks.I have installed hand valves so that the owner can manually defrost coils independant of each other.

This fridge has been there since it was a small corner store but now it is the size of a small supermarket and when they are busy I am sure the fridge is open more than it is closed.

Owner is going to update the equipment and the deli fridge is going to be changed at this time, but I would like to give him a short term solution to this problem.

If you raise the cut in of the pressure switch it does stop the icing but the cabinet temp is too high. If the switch is set to maintain correct cabinet temp the coil ices.

Any help is always appreciated.:)

chillin out
22-04-2006, 02:02 PM
If there are no evap fans then it will not defrost itself.

Why not tell them to switch it off overnight?
Or fit a heater underneath the coil?
Or switch it to hot gas defrost?

Chillin:) :)

windy
23-04-2006, 11:49 PM
I have set these up so many times I have lost count.
As Bango man has said, use a low pressure switch, then you are monitoring what the coil is actually doing.

Set the cut out pressure/temperature at the desired case temperature less another 6 - 10 degrees F, set the cut in pressure at around 38 - 40 degrees F.

The system will then run down to cut out temperature and will stay off untill the coil is ice free.

The actual cut out temperature/pressure needs some trial and error setting as the dynamics of each system are different.

Peter_1
24-04-2006, 06:57 AM
Set the cut out pressure/temperature at the desired case temperature less another 6 - 10 degrees F, set the cut in pressure at around 38 - 40 degrees F.
Thiss is only possible if the condensing unit is located in a place where it's allways more then 2C.

PS Try to put the SI units between brackets for the non IP readers.

Dan
25-04-2006, 01:32 AM
PS Try to put the SI units between brackets for the non IP readers.

What's that about, Peter? I am not sure I follow you.

frank
25-04-2006, 08:18 PM
Hi Dan

here's an example:

Here in the States Copeland data sheets rate condensing units on a chart starting at 90 degrees f (32 c ) and as teperature goes up from there the capacities diminish. So it would seem that would be the place to start. Thanks, Ken

Dan
25-04-2006, 11:26 PM
Oh. At least I understand what Peter was asking for now. Thanks, Ken. I can see it as a courtesy easily provided when referring to charts that provide both measurements, but I think it would diminish my ability to write a quick reply if I had to stop to convert the units that I am familiar with to SI units. I suppose I could print out some spreadsheets and see how difficult it would be to always provide alternate units of measure. Kw/btu, Bar/psig, and F/C. That should be all I need, right? What does Peter suggest?

Vegas
26-04-2006, 11:25 AM
Ken, Try setting the cut in for the Comp too Be above 0deg c say about 2deg c , this affect giving the case a bit off a offcycle defrost when ever it cycles off, 2x 60min defrost is what i set my natural draft delis at...
Cheers Vegas