EPR or Hot gas bypass

[mad fridgie]

Just a quicky, what do you think gives the best "protection" EPR ( + small vapor by-pass)or Hot Gas By-pass (+ control sol valve) on water chillers ( not load control) with the need for pump down

[chemi-cool]

HGBP by far.

I don't like EPR's, they can cause oil return problems if they are not tune well.

Ambient temp influence them too much.

Thats my opinion anyway.

[US Iceman]

Protection from what?

[mad fridgie]

Protection from what?

Sorry for that, protection for the evap, (reduced water flow and/or low water temps) Ensuring freeze up does not occur.
Normally I would see very little difference(even though I do lean to EPRs) but because this needs pump down capability, a solenoid valve has to be fitted in the hot gas line, so I see this as a future problem (sol valve fails) no mechanical protection, EPR I have not had a hermetic one fail fully open.

[Peter_1]

HGBP and a suction accumulator with a TEV injecting its liquid at the inlet and the bulb of the TEV at the outlet of the SA to prevent excessive temperatures of the suction.
An SV at the inlet of the TEV running together with the compressor.
If you want to pump it down then you need a second SV in the HGBP, otherwise the HGBP will bypass if you shut off only the liquid.

[mad fridgie]

HGBP and a suction accumulator with a TEV injecting its liquid at the inlet and the bulb of the TEV at the outlet of the SA to prevent excessive temperatures of the suction.
An SV at the inlet of the TEV running together with the compressor.
If you want to pump it down then you need a second SV in the HGBP, otherwise the HGBP will bypass if you shut off only the liquid.

Yes I agree with this method for capacity control, but is there not more room for failure,when the only reason for installation (last resort) is to protect against other abnomalities within the system, such as low flow and flow switch failure. (common faults)
What would happen if the sol valve to the HGV failed (without being diagnosed) then a year later low flow occurred, would the evap then fail (burst).
The issue is the pump down and the need for a sol valve, in my opinion is the week link, on low level maintained equipment.
EPR I have always found them to be reliable BUT will always cause a small pressure drop thus reducing the long term efficiency, in comparrision to HGV (HGBP)
The only consideration is evap protection under other fault conditions.
The water is used for batch cooling so no need for precise control and flow is "normally constant" On for 8 hours flat tack, off for 4 hours (sleeping)

[Peter_1]

You could also use both: HGBP and an additional EPR as a safety valve.

You could also install 2 LP cut-out's: one activated while normal running and the other switching when you want to pump down.

Why you anyway want to pump down this? Just an SV in line with the compressor. Pumping down with water is always dangerous.

The EPR gives indeed a small pressure drop which will indeed reduce long term efficiency. But what about the HGBP then, is this an efficient method?
You then better reduce your compressor speed or install a digital scroll.

If you have a low level maintained equipment, then there will always be situations where system will failure.
You can't have both: no maintenance and failure safe working.

[mad fridgie]

Hi Peter,again logical points, no need for speed or digitial control of the compressor under normal working conditions load is constant. (large thermal storage)
The compressor is not dedicated to water chilling (hence the need for pump down) Normally I would not recommend pump down.
"Money no object" then having multi layers of protection is always good, but, in this (these) cases only one method is affordable.
So it comes down to the simple question, which gives the best long term protection of the evap under other fault conditions "EPR" or "HGBP & SV" I am not looking for any form of load control!
or Should I be asking has any body had an EPR fail open (failing closed protects the evap)

[RANGER1]

For hot gas bypass , which I assume is piped up directly after TX valve .
If so no need for any controls to keep suction temp down .
Could you instal normally open solonoid valve in HGBP to give it the protection you want . Control of solonoid with relay . If something wrong you probably won't get desired water temp .
There is some pretty good electronic flow swiches available these days as well as digital t/stats .
Also is pump interlocked with compressor .
Another option is different style of evaporator .
I call them baudolot chiller . Basically corrogated plates which water flows over .
No chance of freeze up , but probably cost more .
Everyone seems to go for shell + tube or PHE these days .

What water temp do you want , sounds like a bakery or something ?

[mad fridgie]

Hi Ranger, good points, I normally use a current sensor on the pump as well as the interlock, but such a small pump even with the shaft broken a reasonable current is still drawn.
Have used plenty on baudolot tubes, always a safe option or have used many chester janson plates this when requiring temps close to 0C.
This application is a farm and looking for water minimum stored temp of 6C (entering min could be said to be 6C)
As standard we will have control and safety stats (water in and water out of the evap) flow switch, interlock.
Being on a farm they are basically install and forget until there is a breakdown.
The practical concern is not pump failure, but reduced flow due fouling around the pump inlet. and possible air introduce into the wter circuit. Both of these may not trip the flow switch. causing possiblefreeze up.
If it was not for the requirement for pump down, (pump down is required as it is "possible" to blow the dimple plate in the milk vat, can be upto 85C inside the vat) then i would choose HGBP, but introducing a SV into the Hot gas line you introduce a futher weak point. i had thought about normally open, but without checking they are considerably more expnsive and are not normally a standard stock item (at local branch levels).
Like many things it is price driven, not best engineering. (which is always easy) trying to find the comprimise between cost, performance and longevity (reliabilty) is the trick.
To me a burst evap (water) is the worst possible fault!!!!!

[US Iceman]

...having multi layers of protection is always good..

Absolutely! The difficult part is trying to do without breaking the bank.

For what you describe, I typically use hot gas bypass, which is injected downstream of the TXV, but prior to the liquid distributor. The hot gas mixes with the liquid before all of it flows into the evaporator. Then, if you are using a TXV the superheat is still controlled.

For pump downs, shut off the liquid solenoid valve. Then allow the chiller to pump out the liquid. The hot gas valve should stroke open as the evaporator pressure falls. Once this occurs, the liquid is gone so you can shut off the hot gas solenoid and continue with the pump down.

If you get rid of the liquid, you you hae less issues with freezing.

[oldesky]

I would use a pilot operated EPR valve with solenoid shut off option. If the money is there combine it with electronic TX valve that can be programmed to close below a certain superheat setting.E.G.3K

[Gary]

I would use a pilot operated EPR valve with solenoid shut off option.

I believe what you are describing here is a SORIT valve (combination EPR and stop valve). These are very common on supermarket rack systems and they work very well.

[dryair]

We have historically used HGBP to act as a defrost when needed on our chiller and refrigerated air dryers. I have recently been introduced to the EPR valve. It seems this might be a better match than a HGBP (assuming it's properly configured).

I noticed in the Emerson flow controls catalog - the EPRB description mentions that the "stop" version of the valve (i.e. the EPR the includes the SV) is for gas defrost applications.

So I'm wanting to confirm/kill my assumption of how the EPRS works to defrost the evap. I'm thinking: when defrost is needed, close the SV. Pressure builds up in the evaporator, increasing temperature, and viola, defrost is achieved. Be sure and open the SV before the high pressure switch closes.

[Peter_1]

I personally think the EPR valve is better because your compressor will consume less where with HGBP you artificially load the compressor to design load, whatever load the evaporator is seeing.
Viola c'est mon idée du fonctionnement de la vanne EPR

[Gary]

We have historically used HGBP to act as a defrost when needed on our chiller and refrigerated air dryers. I have recently been introduced to the EPR valve. It seems this might be a better match than a HGBP (assuming it's properly configured).

I noticed in the Emerson flow controls catalog - the EPRB description mentions that the "stop" version of the valve (i.e. the EPR the includes the SV) is for gas defrost applications.

So I'm wanting to confirm/kill my assumption of how the EPRS works to defrost the evap. I'm thinking: when defrost is needed, close the SV. Pressure builds up in the evaporator, increasing temperature, and viola, defrost is achieved. Be sure and open the SV before the high pressure switch closes.

We are in danger of getting seriously sidetracked here as mad fridgie's system does not need defrost. His use of the stop function would be entirely different.

That said, no... the defrost function does not work as you describe in a gas defrost system. The stop valve closes and hot/cool defrost gas flows into the suction line and backwards through the coil into the liquid header (reverse flow).

It's complicated.

On second thought, perhaps using the valve as you describe would work if you don't need much defrosting or are preventing frost... simply stopping the refrigerant flow and letting the water temp drift upwards on its own.

[Gary]

If it was not for the requirement for pump down, (pump down is required as it is "possible" to blow the dimple plate in the milk vat, can be upto 85C inside the vat)

I'm not clear on the need for pumpdown, but the EPR with stop function would seem ideal if liquid refrigerant storage in the chilled water evap is not a problem.

[dryair]

no... the defrost function does not work as you describe in a gas defrost system. The stop valve closes and hot/cool defrost gas flows into the suction line and backwards through the coil into the liquid header (reverse flow).

Thanks, that makes sense.

[dryair]

I personally think the EPR valve is better because your compressor will consume less...

Agreed. I have always looked down on HGBP because of this.

Sadly, I had been unaware of the EPR until recently (like this morning). I don't feel too bad though, I wasn't brought up as a refrigeration engineer - I inherited the position by default when our origional refrig engineer left a year ago!

[Mike NZ]

Hot gas bypass injecting into a side entry distributor fitting just after the TEV. size the HGB for 50%. the hot gas solenoid coil should be suitable for dissipating the extra heat. EPR will cause oil logging and insufficient compressor cooling as the heat load is not constant through the evaporator, hence the need for a certain amount of capacity control via HGB.