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chillyblue
17-09-2007, 06:40 PM
Hi

I have noticed something today which i have not experienced before.
A system operating at -20 with R404A on a DX internally equalized TEV.
As soon as it went onto hot gas defrost, the compressor became very very cold and even started to freeze over slightly it then gradually cleared throughout the duration of the defrost.
The hot gas line enters the evaporator after the TEV, with a check valve stopping hot gas going back to the TEV.
The sequence of the hot gas defrost is
1 normal refrigeration, sol valve in liquid line open, hot gas sol valve closed, evap fans on.
2 hot gas defrost, sol valve in liquid line shut, hot gas sol valve open, evap fans off.

I can only assume that the hot gas has blow any liquid sitting in the evaporator straight out, but why haven't i noticed it before.
Any ideas

Cheers
CB

cameron.e
17-09-2007, 07:10 PM
Hi Bud Sounds Like The Condenser Fans Is Not Going Off On Defrost Or It Is Over Condensing,due To Low Ambient Temp,cheers

Grizzly
17-09-2007, 08:37 PM
Hi Chillyblue.
I would agree with your initial assessment that liquid is being carried over from the evaporators.
Several possible reasons spring to mind.
1) Is there a time delay between the liquid line solenoid valve closing and the hot gas valve opening.
This delay should be sufficent to pump down the system removing any liquid sat in the evap. It is also possible that the evap. fans run on during this time to create more heat load thereby assisting the liquid removal.
2) Is there a blockage across the evap, reduceing air flow and heat load across the evap. Resulting in liquid trapped in the evap not boiling off properly. This is quite common on large cold store type evaps. Where greater than normal moisture has been introduce into the room ( someone left the door open). Causing the evap to freeze up heavier than normal and then in turn not getting cleared by the normal defrost duration. When this happens it's quite common to find the evap covered in "Snow".
3) A passing liquid line solenoid valve could cause this problem.
4) A quick check of the superheat on the compressor suction inlet might clarify things a little.
These are all general questions of what is happening
and may or may not fit your scenario. If I am teaching you to "suck eggs" I appologise.
cheers for now Grizzly.

chillyblue
17-09-2007, 09:01 PM
Hi Grizzly

No there is no delay between the liquid valve shutting and the hot gas opening.

I think the same as you, i need to pump the system down first to clear the evaporator.
thinking of other hot gas systems i have seen, they usually have a suction accumulator i assume to take any flood back, This system is only a small 1.5kw system and has no accumulator

Cheers
CB

Grizzly
17-09-2007, 10:05 PM
Hi CB
It worries me that there is no time delay. How else do you eliminate the possibility of liquid carry over.
Although it has just occured to me that some hot gas defrost systems don't have a time delay as such. The hot gas/ Suction return is metered through a PM valve regulateing the suction return pressure.
resulting in any liquid being boiled off and returned through the PM Valve as vapour.
The different options are endless at the moment.
But it will be well worth following up as to why this is happening.
What type of control is opperating the defrost cycles?
Could you put a delay-on timer in to eliminate the problem ect.ect.
It sounds stupid but how clean is the evap?
Sorry I can't be more positive but it's difficult to second guess what is happening.
But you are definately on the right lines.
Cheers Grizzly

chillyblue
18-09-2007, 07:08 PM
Hi Grizzly

Thanks for the help, i have advised my client to put a 3 min delay into the system at which time both the LLSV and Hot gas valves will both be shut, allowing the unit to pump down, after the 3 mins the hot gas valve will open to defrost the coil.
He needs to mode the program as it is on a PLC.
When he's done i shall see what happens.

Thanks again
CB

Grizzly
18-09-2007, 09:43 PM
Thats great news CB. I don't know if it's possible or that you have allready thought of it. But if it's not embarrasing to go back to the customer or their BMS operatives.
You might like to consider the other defrost control parameters usually found in a defrost stratagy?
1) liquid line sol.valve closes (as allready discussed)
2) evap fans stay on to help boil off any remaining liquid in the evap.
3) Evap fans go off.
4) H. Gas sol. valve on and on for its preset duration.
5) H. Gas sol valve goes off
6) Liquid line sol. valve opens
7) after a drain down period (to allow any residual heat left in the evap to be absorbed by the incoming liquid refrigerant)- say 60 secs evap fans are started.
this also reduces the likelyhood of any excess heat being blown into the room on evap fan start-up.
A pleasure to be of assistance and hpoefully some of this has been a help. No egg sucking intended!