R717 Refrigeration System with Thermosiphon Oil Cooler - Trouble Shooting

[RANGER1]

Pratap,
Initially thinking air in system.
As long as liquid receiver level no higher than 80%
Turn plant off, keep condenser fan & water spray pump going for 1/2 hour.
Isolate compressor discharge stop valve, valve 12 balance line then with hose in water purge aggressively from valve 18 off top of condenser.
You could also purge off top of liquid receiver separately.
You may get liquid ammonia out after awhile as it symphony’s up into condenser.
You may see air bubbles rise to top of water, keep plenty fresh water in bucket, or drum.
You may have to do several times.
Normally a purge valve is welded on top of condenser liquid drain to purge air when running.
Air can be detected by using saturated discharge pressure example 35 deg C 1250kpa.
If liquid drain is colder than that, likely air in system.
Let us know outcome.
Assume fan has good airflow, pump sprays flooding condenser coil with water, liquid receiver is about 1.5 metres from liquid drain to liquid receiver level to allow draining.

[Pratap]

@RANGER1,

First of all, I thank you for your reply. I have been evacuating air everyday for 30 mins before startup though I have not kept the condenser on. Shall try that out.

However, from my experience, an increase of 2-3 barg discharge pressure is rarely due to air trapped inside. I anticipate a design issue, may have something to do with letting the liquid flow directly into the oil cooler. Is there a possibility of the 80 NB gravity line being partially filled with gas and may need equalizing or something ?

Have you seen anywhere, the condensed liquid from the condenser directly taken into the Oil Cooler and not through a priority vessel or a raised receiver ?

[RANGER1]

@RANGER1,

First of all, I thank you for your reply. I have been evacuating air everyday for 30 mins before startup though I have not kept the condenser on. Shall try that out.

However, from my experience, an increase of 2-3 barg discharge pressure is rarely due to air trapped inside. I anticipate a design issue, may have something to do with letting the liquid flow directly into the oil cooler. Is there a possibility of the 80 NB gravity line being partially filled with gas and may need equalizing or something ?

Have you seen anywhere, the condensed liquid from the condenser directly taken into the Oil Cooler and not through a priority vessel or a raised receiver ?

Yes, have seen systems very similar, in your case liquid drain level from condenser would be at same level as liquid receiver, which should be at least 30% level.
Cannot see obvious issues
Unless you close the valves I suggested, you may not get air out of condenser, but just blowing off gas from receiver & associated pipework.
Basically isolate condenser, then purge off top with fan & pump running.
Got to 100 % eliminate air before considering design issues that may not be problem,as that becomes more difficult to change.

[Pratap]

Noted. I will do as you suggested and revert. Thanks.

[RANGER1]

Pratap,
Check your email.

[Josip]

hello Pratap,

bellow is your lost post from your thread I was trying to merge all posts but no way ... not sure why is not visible with other posts between you and Ranger1 .... anyhow it is here... letters and scheeme
**********************************************

Hello All,

Preamble: I am in the middle of Commissioning a R717 Screw Refrigeration System with a Thermosiphon Oil Cooler. I currently have a problem of high discharge pressure in the Evaporative Condenser which is higher by 3 barg than it should have been for which I seek help from this forum. (Current 14 barg, expected 11 - 11.5 barg).

The Thermosiphon Cooling I have used is slightly different from the standard P&ID used. In that, I have drawn a pipeline directly from the Evaporative Condenser (6 meters above the Oil Cooler) to the Oil Cooler rather than through a Priority Vessel with a tapping to the HP Receiver which is placed at a height of 1.5 meters from the Oil Cooler. The refrigerent is vented out into the reciever which is at a height of 1.5 meter from the Oil Cooler. The receiver is equilized with the Inlet of the Condenser.

The idea behind doing this was to use the entire 5-6 meter height between the Oil Cooler and the Condenser to generate more gravity pressure to cool the oil which I thought would improve the cooling in the Oil Cooler due to higher velocity. With this arrangement, I don't have a problem with the temperature of the oil, however the Condenser Pressure is higher by 2-3 barg.

An increase in the Discharge Pressure was anticipated, so a tapping was taken to the bottom of the receiver to collect the excess refrigerant into it directly thinking it would free the condenser coils of the liquid due to the pressure drop in the Oil Cooler, but this doesn't seem to be working. The Discharge Pressure remains the same even when I open the valve to the receiver.

Attached is the P&ID for your understanding. I wish to have an opinion on where I have gone wrong and what could be a possible solution to this issue I am facing.

Regards,
Pratap

Attached Images


From here is my post i.e. answer to you....


So: I do not like to criticise people becuse they do not know something (I was young too and I did not know a lot of things), but usually people first did the things wrong and then revert for help ... what is not a right way and sometimes can be expensive too ...

first we go to school to learn something and then we start to make things or maybe I am wrong ... anyhow ....

So according to your scheme liquid line is OK ( liquid is draining into oil cooler and surplus is going into receiver .... but return line from oil cooler must go directly into condenser inlet pipe from above side and not into receiver because receiver is not designed for that kind of return line connection ...

went line from receiver must be connectet also in that way ... so you will have two
went pipes connected into condenser inlet pipe from above ... I have to find a scheme how to execute that connection ... but for sure that must be within my RE forums posts ...

another big problem can be ... according to your scheme you have double coil condenser and your outlet from condenser is not good ...

each outlet must be with its own siphone to balance coils ... and then both outlet after siphone connect into pipe going down to oil cooler ... the elbow on that pipe (right side on your scheme at bottom) must be the lowest part of that pipe and from that elbow pipe must climb up into oil cooler ..

also on horizontal outlet pipes from each condenser coil but before elbow going down you must instal purging valves 3/8" on the top of both pipes ... there is only place where you can purge air from your coils ....


further ... kind of siphon ahead of condenser inlet must be upside down ... i.e. inlet pipe to condenser must be with slope to condenser ...
condenser inlet must be the lowest part of inlet pipe ...



This is all for the moment ... hope to be of some help ....
if you have any question please ask


Best regards, Josip

[Josip]

Hi, RANGER1,

I made mistake ...
somehow I was thinking this is Pratap's thread ....
but then I found out it is yours ...

I do appologize to hijacking

... I was trying to help Pratap with his problem ...
he started a new thread with the same name,
"R717 Refrigeration System with Thermosiphon Oil Cooler - Trouble Shooting"
but under NH3 and this is under Refrigerants
seems the same name of the thread was misleading me ...

Again, sorry for mistake.

Best regards, Josip

[RANGER1]

Josip,
I'm not sure what happened now, but I think Pratap edited posts as well removing P& ID.
Not heard back, so who knows!
The type of system he has shown is very common, except thermosyphon always has it's own supply to oil coolers, because he said working, was not worrying about it.
I guess because P & ID not exactly what is real, valve in liquid drain should be in vertical line, or on it's side if desperate, as could hold up liquid in header.

Thanks

[Josip]

RANGER1
we can wait to se if Pratap will return ...

For sure scheme is not the best but we do not know
nothig about real installation ...

I believe he is busy with commissioning ...
so give him some time


Best regards, Josip