Suction valve position when set point is reached ?

[Lambo]

Hi,

In ammonia pump system with hot gas defrost, we have valve on liquid and suctions side. When we need cooling both of this valves are open. When set point is reached, liquid valve is closed but what is your experience with suction valve. Do you keep it open or closed (assuming we have some relief valve like Danfoss BSV) ?

Regards

[RANGER1]

Lambo, either way ok as long as any excess pressure build up is relieved.
Usually use CVP for this.
If you have a lot of units pumping down HP liquid receiver could overfill.
If you want room to control precisely keep full.
Can you explain what you are doing in a bit more detail.

[PaulZ]

Hi Lambo
Agree with Ranger's comments.
I would shut the suction when the rooms are satisfied. If you pump the evaps out and you get several rooms come on together the you could end up with a low level in the accumulator and possible pump cavitation. This could bring up unwanted alarms.
Regards
Paul

[Grizzly]

Hi Guys.
I am slightly confused as to what actual system Lambo has.
Only on the systems I am used to looking at a pumped system has Coolers which operate on a flooded principle.
Which when stat is satisfied the Liquid line Solenoid valve shuts the fans stay running as does the WSSV.

The cooler is slowly emptied by the wet return action / heat load on cooler!

Once set point rises the liquid line solenoid valve opens and a SMOOTH transfer of "Wet Vapour / liquid is maintained at all times.

Resulting in a stable liquid level / Suction Temperature control of the plant within the surge drum.
On Hot gas defrost it is indeed closed and a regulated bypass is then used to bleed the refrigerant / Gas back via the wet suction line.
Obviously It depends what system we are all talking about, but I would not expect the suction Valve to shut
along with the Liquid line valve on a normal pumped Ammonia System.

What do you think?
Grizzly

[Lambo]

Hi Guys,

Thanks for answers. To clarify little more, it is pumped system with hot gas application like on picture:

15174

Question is do we close GPLX (ICLX on know days) when temperature is reached?

In my installation i close it, but I heard that some installer do not closed.

[Grizzly]

Hi.
My initial thoughts are to close it when on defrost ( H.gas would bled back through OFV) but not when stat satisfied.
That way the system operates smoother with less surges back to the Surge drum (Seperator)

I must admit the term evaporator confuses me, if it is as you say a pumped system?
I was always taught a Flooded HE is a cooler (Plate pack) and a DX is an evaporator.
Add a wet return and its definitely a cooler.

I will be intrigued what others make of your system my friend.
Interesting!

Cheers Grizzly

[Grizzly]

So, if the suction valve is shut when stat satisfied and internal pressures rise.
If whilst in this pressure elevated state the cooler went onto a timed defrost.

Would the differential pressure of the hot gas line verses the raised suction.
Reduce the defrost effect and increase the length of time necessary to clear the cooler?

We also use OFV's and have modified their use as.
By utilising the Hot gas bypass solenoid valve (Equalising valve post hot gas injection)
This now opens at the same time as the hot gas solenoid valve.
Significantly reducing the defrost duration and heat load being introduced into the cooler / Chamber area's.

They take quite a lot of Back pressure to force open meaning, a large part of the defrost period can be spent trying to use the hot gas to drive the remaining liquid trapped within the cooler.

Which in itself is another reason to have the suction return valve open prior to defrost.

Just thoughts guys!

[RANGER1]

If on a new system in some cases 2 speed or VSD fans slow down & everything works as normal, meaning LLSV & SSV open.
This way air cooler unit is idling, fans saving energy, waiting for temperature increase to ramp fans up.
As Grizzly mentioned at end of defrost valve opens with potential big rush of pressure.
ICLX is the better choice for suction as 2 step opening after defrost (GPLX is not made anymore I don't think, or at least not prefered).
Sometimes depends on how much money you have to spend, bad experience etc as to how you pipe up.
If you pipe up to Danfoss recommended, then maximum use of valves in some cases.
Some plants are unique, as liquid recirculation vessel maybe to small, liquid receiver to small or runs low/high liquid level.

If you only had a few air coolers on same duty, raising suction pressure can help as well to keep plant running efficiently.

As far as terminology air cooler, flooded heat exchanger, evaporator, if liquid recirculation I get what you mean.

[RANGER1]

PAfew links on hot gas defrost.

http://www.danfoss.com/technicalarticles/rc/hot-gas-defrost-of-low-temperature-refrigeration-evaporators-with-natural-refrigerants/?ref=17179879561#/

http://www.coolpartners.dk/wp-content/uploads/2012/12/LIQUID-HAMMER-VERSION-2-NOV-2000.pdf

http://www.colmaccoil.com/media/42058/optimizing-hot-gas-defrost.pdf

http://www.csb.gov/assets/1/19/final_CSB_CaseStudy_Millard_0114_0543PM.pdf

From last link it may be better to pump down if system controls not set up to recognise different scenarios.
If liquid & suction were closed because room down to set point, then hot gas cycle should still force open suction for pump down before hot gas solenoid opens.

[Tycho]

For this set up, I would do it like this:

Setpoint reached:
1.Close liquid supply
2.GPLX remains open
3.fans continue running

Defrost:
1.close liquid supply
2.wait 2-3 minutes
3.open hotgas
4.wait 5 seconds
5. close GPLX


Start cooling after defrost:
1. open GPLX
2. wait 2-3 minutes
3. open liquid

The GPLX is a 2 stage valve so it won't open fully until the differential pressure across the valve is low enough for the hotgas pressure to lift the main valve seat.
This is also why you don't open the liquid valve until the GPLX has fully equalized the pressure, if you introduce liquid to soon, the increase in pressure in the cooler will make it take longer to open the GPLX.

On set ups like this we usually have a smaller solenoid valve bypassing the main hotgas valve to increase the pressure a bit before the main hotgas valve opens.

[Grizzly]

Spot on Tyco.
Replace the GPLX for more modern valves and what you describe is Pretty much what I am used to seeing.

Grizzly

[RANGER1]

Tycho,
Back from that holiday you had!
Pretty standard for hot gas defrost.
But what about original question of closing suction with liquid when room reaches set point?

[Josip]

Hi all,

I do not want to say that I am very old :D

... but similar set up with some other Danfoss valves (available in that time) we installed decades ago

... in general GPLX can be closed in the same time with ICS3 or by timer few minutes later ... any increase of pressure within evaporator will be equalized via OFV15 or via BSV ...


I do agree with Tycho's suggestion about setup ...

1.when set point reached we can close liquid line ... suction valve remains open and fans running to mix air within cold room ... or by room thermostat shut down fans too and keep GPLX open or closed ... maybe better to be closed, because we had one step less in control sequence (GPLX must be closed during defrost) ...

All depend on available timers and possibility to use them ... but if all is working good keep your setup i.e. close GPLX valve together with ICS3 ...

The best is always the simplest solution ...


Best regards, Josip

[Segei]

I worked with systems which keep suction valve open when temperature is satisfied. Liquid solenoid off and fans off as well. Running fans will waste energy. Actually liquid will gradually evaporate due to air natural convection. If suction valve will be closed, pressure in the coil will increase by 10-15 psig. No problem if suction valve gradually open when cooling mode initiated. My concerns about hot gas defrosting. It should be not initiated if liquid present in the coil. It is ok if coil pumped out before defrosting.

[Tycho]

Tycho,
Back from that holiday you had!
Pretty standard for hot gas defrost.
But what about original question of closing suction with liquid when room reaches set point?

Keeping busy Ranger :) Been in the middle east for 3 months on a ozzy made compressor skid to assist with refrigeration knowledge during commissioning :)

About closing the suction valve when the temperature is reached, I don't really see the point in this.
The less you interfer with the refrigeration cycle the less problems there will be.

The liquid line solenoids are more prone to get dirty or suffer failure, even if there is a filter upstream of them, and to have the suction valve closed if there is an internal leak in the liquid valve is just asking for trouble, even if there is a OFV valve.

In general, glosing the suction valve is only done during defrost or service, closing the liquid valve is the norm, and the system should of course have been designed to accomodate any excess liquid in either HP or LP receiver.

in my opinion ;)

[RANGER1]

Fair enough.
Hope the Aussie stuff was up to standard!
Made in Australia via Indonesia or Malaysia :)

[Lambo]

Thanks for this answers :).

I just wanted adder opinions. Some conclusion will be:

1. When it closed, then compressor work less because refrigerant is not emptied from evaporator, but GPLX (PMLX or ICLX) work harder and can break more frequently. Temperature control is more precise.

2. When it open, everything opposite then stated by 1.

[Tycho]

Thanks for this answers :).

I just wanted adder opinions. Some conclusion will be:

1. When it closed, then compressor work less because refrigerant is not emptied from evaporator, but GPLX (PMLX or ICLX) work harder and can break more frequently. Temperature control is more precise.

2. When it open, everything opposite then stated by 1.

to some degree.

if the valve stays open on reached set point, the compressor really wouldn't work any more than it should because the room is at temperature. if you stop the fans there would be no heatload on the cooler.

if you close the valve there is going to be pressure buildup in the cooler and when the valve opens it's going to cause a surge that the compressor can't keep up with, and it's going to have to work harder to get the pressure back down.
as opposed to keeping the suction valve open, there wouldn't be any significant surge when the liquid valve opens, and so the compressor will have an easier task of maintaining the suction pressore.

imo

[Tycho]

Fair enough.
Hope the Aussie stuff was up to standard!
Made in Australia via Indonesia or Malaysia :)

Designed in Australia and built in glasgow by Howden I think, quite nice skids. There's a commissioning engineer from Howden here dealing with the mechanical part., but they don't do refrigeration apparently so I'm here to handle that part.
Haven't had any heat load on the chillers yet so we have only ran one of the compressors for 40 minutes to move propane out into the system. Still waiting for the client to give us access to wet gas. :)

[Superfridge]

Tycho is on the money from my experence. Also with larger suction valves it could take up to 1000 seconds to close then open the suction valve and the extra wera and tear on the valve to take into account. If I owned a plant I would rather pay for a smaller liquid supply valve service than a large sution valve. I have seen mostly suction valves controlled via a Danfoss CVQ and EKC.