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Dossat
04-06-2014, 10:19 AM
help please!
Got a site where the Gravity feed Ammonia Plate pack is not working.
Checked for blockages on Ammonia side & Glycol side of the Plate pack and no blockage its clear of oil etc.
We have shut off the glycol and witnessed the plate frost line fill the plate on the ammonia side.
We have correct flow rate & pd on the glycol side so plenty going through & good level in vessel.
The only way of getting any cooling at all on the glycol side is to close down the glycol to 30% of its normal flow rate. As soon as the glycol flow is increased the TD across is lost.
As I'm running out of ideas I was wandering if the level in the vessel is too high and has risen above the ammonia return outlet in the vessel in effect back filling the plate with liquid.
I know its doubtful but has anyone experienced this issue before? Would the thermosyphon effect stop as the plate is being filled all be it with gravity fed liquid ammonia but to inlet & outlet of heat exchanger, causing it not to do any work?

RANGER1
04-06-2014, 10:38 AM
Dossat,
Just a few things
What is saturated suction pressure?
What is required glycol temp?
what is glycol freezing point, has it been checked?
Is it new, or has it worked in the past ?
If you pump plate pack out check for dead ammonia or water.
raise or lower liquid ammonia level to see if it performs better.
What does the glycol cool, any products that could leak into glycol?
How are you reading level in ammonia surge drum bulls eyes, level probe, both, other?
Does level control column have any dead legs to possibly fill with oil, giving false level?
Do you see any load on the compressor side of things?

Dossat
04-06-2014, 03:07 PM
suction 2.3b, glycol -8C, percentage 34%
No leaks pump pressures up to what they should be.
Not new, has worked before and in fact is 1 of 2 PHE's on a common system the other is working ok.
Levels are on Bulls eyes and a level probe.
Compressors are doing what you would expect for only one of two plates working.
Thought we would find a blockage somewhere on the plate pack but no.
Put hose on Ammonia return to drum to see if vapour locked and didn't really improve that much at all.
I know if it were the ammonia levels in the drum you would think both plates would be affected however it is a large drum levels may differ from PHE to PHE & my question regarding thermosyphon working if it did back fill.

Dossat
04-06-2014, 08:10 PM
sorry, suction is 1.8bar!

RANGER1
04-06-2014, 09:24 PM
I have seen them overfull & still working!
That doesn't mean yours will be the same.
Experiment, lower, raise levels & make sure control column probe & bulls eyes react accordingly.
One in particular I worked always read 13% & not flooding, but surge drum so oversize starved the rest of the plant for liquid. Ended up poor design & control column dead leg full of oil.
All I can say is assume nothing.

Tycho
04-06-2014, 09:26 PM
At first when reading your post I thought "oil in the system" but that has been taken care of, so :)

my first thought when something on a plant stops working, and there are two of them is "is there any difference in how the pipes are laid?"

Could you possibly draw up a schematic of how the pipes are arranged, from the ammonia drum and down to the two PHE's?

I'm guessing the PHE's are standing and that the ammonia is entering through the lower connection and should boil off through the top connection, while the glycol is entering on top of the PHE and exiting at the bottom to get "counter flow".

You say the glycol is -8C, is that the desired temperature or the temperature of the glycol entering the PHE?
another thought that popped into my head was that if the ΔT between the ammonia and glycol was too high it would cause excessive boiling in the ammonia and stop the "circulation" of new refrigerant.
I started leaning a bit more towards this after you said that it works if you reduce the glycol flow.

are there any shut off valves on the delivery or return line that might be broken or restricting flow?


I don't have any experience with thermosyphon and PHE's, but I have been on a few York systems where on the worst one I drained over 300 liters of oil from the oil cooler since there was a new machinist and he hadn't been informed to drain oil from it :) (you said no oil, just thought I'd mention it for future reference in case anyone searces for a similar problem :))


Anyways, a diagram showing how the pipes are on the two PHE's would be nice to make a more educated guess :)

Tycho
04-06-2014, 09:27 PM
Also, what Ranger1 said :)

RANGER1
04-06-2014, 10:29 PM
To drain oil suggest pump down completely & have to drain from lowest possible point

sandybapat
10-06-2014, 07:03 AM
The Ammonia liquid head should be sufficient to overcome the pressure drop in the vapour return line. In case the liquid head is less, Ammonia will not flow through chiller and heat exchange will not take place. May be with 100% glycol flow the pressure drop in vapour return line is higher and Ammonia flow is restricted. When the glycol flow is reduced the rate of Ammonia evaporation is lowered and the pressure drop in vapour return line just matches the liquid head and Plate pack works with 30% capacity. I suggest increasing the vapour return line size to reduce the pressure drop and if possible to increase the Ammonia liquid level in the surge drum.

There is a problem with very high liquid head also. In this case the Ammonia liquid entering the PHE is sub cooled. The liquid first gets sensibly heated to boiling point and then starts evaporating. Lot of heat transfer surface of PHE is utilized in this sensible heating and this affects the PHE capacity drastically.

PaulZ
11-06-2014, 12:05 AM
Hi Dossat
As Tycho has asked a drawing would be good.
I take it there are 2 PHE's attached to the same drum, is this correct or do they have their own drum?.
Does each plate have a separate oil drain?
Have you been draining oil while the PHE is running?
Does the bottom line to the PHE slope back to the drop leg or to the PHE?
Is the oil drain in the drop leg or in the line to the PHE?
A photo would be good.
Regards
Paul