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. :eek:

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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Originally Posted by ecclesk

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. :eek:

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If suction closes, how is hot gas/condensate pumped out of evap?

Rog

a pressure regulating valve bypasses the suction soleniod, i'll try and post a drawing later/tomorrow

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Originally Posted by ecclesk

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,

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If you will pump it down and then aftewerwards injects hotgas in it, then this will be a big pressure rise at once, a pressure shock.
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 ?

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Originally Posted by Peter_1

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.

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Well said, Peter! I agree completely. It is often an expensive lesson to learn: "If it ain't broke, don't fix it."

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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What is wrong with injecting hot gas in a not evacuated evaporator?

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If you stand in the roof space next to a valve station above an evap when a defrost starts you can hear the evap creaking, banging & making lots of noise, also for the first 10 or so minutes of the D/F the pressure reg valves hammer like mad, and damage the teflon seats and vibrate pipe work i dont think they like the mixture of vapour & liquid passing through them at the same time. once the teflon seats are worn or damaged we then have the problem of the pressure reg valves allowing liquid to bypass the evap which casuses more issues.

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

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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.

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we do have a small bypass line over the main suction SV with a hand reg halve on it this is open around 2 1/2 turns.

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Perhaps stage 4, first open suction valve ad then afterwards the liquid line.
Is the suction valve a NO one?

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suction SV is normally closed.

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Just for my own interest, how do you remove the oil out of the liquid ?

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We have no oil separators on compressors, a small amount of the liquid being pumped from surge drum to evaps is diverted to a heat exchager on the discharge line, a TEV controlls feed into the HX, refrigerant is vaporized seporating it from the oil, the outlet is connected to the commen suction for compressors.

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).

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?