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Thread: Defrost sequence
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23-02-2005, 05:01 PM #1
Defrost sequence
I look after a cold storage site, three stores each has its own plant room. each plant is pretty much the same set up, R22 pump curculated, flooded evaps, 2 stage compound compressors, hot gas defrosts (pressure regulators on evaps), PLC controlled. Grenco installs 130, 195 & 288kw
for some reason when installed the liquid & suction soleniods on each evap are controlled by one PLC output so they are either both on or both off, this means it's not posible to have a pump down before the evap is defrosted, in other words hot gas straight into a evap full of liquid.
Finally got the go ahead from the boss to spead some money and get this sorted, i'm going to fit output expansion blocks to the mitsi PLC's and add extra relays so the suction and liquid soleniods can be independantly controlled.
at the moment the sequence is as follows:
Stage 1, defrost initated fans stop perif heaters on, liq & suc closed, hot gas open (35 mins)
Stage 2, drain down, hot gas closes (4 mins)
Stage 3, snap freeze, liq & suc open (4 mins)
stage 4, fans on perif heaters off, back to normal running,
i'm looking to change it to:
Stage 1, pump down, liq closes (4 mins)
Stage 2, defrost, fans stop, perif heaters on, suc closes, hot gas opens (35 mins)
Stage 3, drain down, hot gas closes (4 mins)
Stage 4, snap freeze, liq & suc open (4 mins)
Stage 5, fans on, perif heaters off, back to normal running,
Im I on the right lines with the sequence above?
Does any one have any advice for the ideal sequence for this system?
The timers will all be ajustable via a HMI.
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23-02-2005, 05:13 PM #2
Re: Defrost sequence
Originally Posted by ecclesk
Rog
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23-02-2005, 05:20 PM #3
Re: Defrost sequence
a pressure regulating valve bypasses the suction soleniod, i'll try and post a drawing later/tomorrow
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23-02-2005, 07:46 PM #4
Re: Defrost sequence
Originally Posted by ecclesk
We have +/- the same set-up and they do exactly the same sequence as your actual sequence.
What is wrong with injecting hot gas in a not evacuated evaporator?
Why do you wnat to change it? Are there complaints of anything? If something has worked for a long time without big problems, my experience is ...leave it that way.
The liquid will be pushed out anyhow to the receiver.
We install mostly a smaller size SV (3/8) over the main suction SV valve and open the small one first to release slowly the high pressure into the suction side.
Perhaps stage 4, first open suction valve ad then afterwards the liquid line.
Is the suction valve a NO one?
What do you mean with 'snap freeze'?
Just for my own interest, how do you remove the oil out of the liquid ?Last edited by Peter_1; 23-02-2005 at 07:52 PM.
It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.
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24-02-2005, 12:17 AM #5
Re: Defrost sequence
Originally Posted by Peter_1
I "improved" a few systems in the beginning, only to find out that there was a reason things were done "wrong" on certain systems. It's a great learning experience, but expensive in time, if not money.
That's not to say there aren't real problems with some systems, but if it's been running fine for a long time with no problems....
Rog
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24-02-2005, 12:57 PM #6
Re: Defrost sequence
What is wrong with injecting hot gas in a not evacuated evaporator?
also im hoping to reduce D/F times, at the moment we have say
35 mins of hot gas but the first 10 or so mins no defrosting is taking place as the hot gas is fighting agaist the liquid in the evap. i'm thinking that when that mod is complete that the pump down will mean the the defrosting will be more efficient
We install mostly a smaller size SV (3/8) over the main suction SV valve and open the small one first to release slowly the high pressure into the suction side.
Perhaps stage 4, first open suction valve ad then afterwards the liquid line.
Is the suction valve a NO one?
Just for my own interest, how do you remove the oil out of the liquid ?
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11-06-2006, 07:50 PM #7
Re: Defrost sequence
1 why change if working
2 i would not change program as grenco will not be able to service it anymore.
3 is liquid in evaporator going up or down? if goinbg up the evaporator will be full of liquid after defrost.
4 i would only use a small timer for keeping the liquid valve closed for 30 seconds. I have lots of on/off controls just working with a simple timer.
5 control with HMI costs and is complicated, dangerous as anybody can change it. GRenco will not service it anymore.
try to convince grenco to do it in cooperation and i do know they will listen to reason as they can make money out of your idea and get it from you (the money).computer shooter
paul deelen
+31653300739
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25-06-2006, 04:52 PM #8
Re: Defrost sequence
Dear experts,
Is there any new applicable technic or somehow usual device to recognize the defrost starting time of a subzero evaporator in cold room or in blast freezer?
Although I have heard a bit little somting about :
*photo-electric eyes switch
*ultrasoanic device
*barometric pressure-state&with delta(T) controller in PLC
I would be thankful if I can get advises?S.P
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