Suction Line Accumulator

[bersaga]

We are in the process of making some standard low temperature R-22 condensing units or CU's ( application T evap between -40 C to -15 C)

We are having a debate whether to put in a Suction Line Accumulator.(SLA)

One argument is for the SLA : Reason - we do not know how the contractor who buys the CU matches it with the load, maybe an oversized TXV, leaking TXV,overcharging etc. We have to protect the compressor and if the CU fails because of floodback, the contractor will blame the CU supplier and not himself.

The other thought is for eliminating the SLA :Reason - we have no control as to what the contractor does on site and cannot cover every possibility- and also how big a SLA would you put - what % of the total charge ? We also do not know at what evaporating temperatures the actual application is for.

The other reason for the SLA is (from a commercial point of view) is that there is no confidence level from a customer/contractor if a low temp system does not have a SLA.

I need all your advice , especially those you have installed and serviced low temp R22 DX systems.

Thanking you in advance.

[Dan]

Bersaga, your question is good, even though it is not a single question.

A suction line accumulator should be sufficiently sized to hold the entire charge of the system. Sounds dumb, eh? Well it is not a bad starting point for detailed discussions.

Why on low temperature applications? People always want a suction line accumulator on low temperature applications. Perhaps it is a leftover misapprehension from those who fear ice on end-bells.

I am just guessing here, but I think that the desire for accumulators on low temperature applications is related to the greater likelihood that the evaporators will frost up more completely when something goes awry, such as a door left open, or a time clock failure, or a termination thermostat failing... etc.

That the TEV will permit liquid refrigerant into the suction line.

In such cases, I would think that you would want an accumulator as large as the refrigerant receiver. I have seen suction accumulators fill up with liquid and become useless. I have seen some not fill up and appear to serve the system as a legitimate safety, too.

A small accumulator is the equivalent of a hood ornament on a car...perhaps affording symbolic comfort. Which can address your commercial concerns, without necessarily addressing the purpose of the accumulator.

Pardon my blather. But I think I sorted out a question or two that is worthwhile:

For what failure mode do we expect the suction line accumulator to provide a safety?

Actually, that's the only question I can think of.

[Gary]

In general, the purpose of accumulators is to catch the overflow where there is a fluctuation in load that the metering device is unable to compensate for. Dan gives a good example with the frozen coil scenario. You might view accumulators as insurance.

Unfortunately, many view accumulators as a "cure" for an ongoing problem. In reality, they only cure the symptom of the problem. They are all too often used to compensate for bad design and/or failure to find an underlying problem.

[bersaga]

Hi Dan/Gary !

Thanks for the replies.

I believe SLA selection is not a science but more field experience - what could be the SLA be selected for - 30%- 50% of refrigerant charge or something like that ? I guess I'm trying to gauge a industry rule-of thumb. I know Carly selection is based on min/max loads and operating evaporating temperature. Alco just tells you for a particular evaporating temperature , a particular SLA will hold x amount of liquid. All this presumed that the system is designed and components selected well in the first place.

[terrygoodrich]

It does not seem likely that a component failure would cause a floodback of the entire charge. The accumulator is only a backup safety device to protect the compressor. An accumulator of any size affords better protection than no accumulator. An R-22 system operating at those temps will have a huge compression ratio and run very high discharge temps. The accumulator will add some superheat to it to make matters worse. It would be a good idea to add liquid to the accumulator inlet with a TXV to desuperheat the suction vapor.

[Dan]

It does not seem likely that a component failure would cause a floodback of the entire charge.

I disagree. a 7/8 OD gas defrost bypass check valve hanging open pretty much is a short circuit between the liquid and suction lines in a supermarket application.

It would be a good idea to add liquid to the accumulator inlet with a TXV to desuperheat the suction vapor.

I sort of follow your thinking, but injecting liquid into a device designed to catch liquid overflow has its own irony.

I don't argue that an accumulator can be a good thing. I just wonder what it is that we are attempting to compensate for by adding it to anything. It is not really a protection device like a circuit breaker in an electrical circuit.

Domestic heat pumps have an accumulator, for example, that is sized sufficiently to hold the entire charge. I imagine there is a good reason for it, although I am not sure what that reason is.

The size of an accumulator is based upon the size of the error, and the likelihood of the error you think can occur. To misquote a line from "Cool Hand Luke":

What we have here, is a failure to accumulate.

[herefishy]

Originally posted by Dan
[B]I sort of follow your thinking, but injecting liquid into a device designed to catch liquid overflow has its own irony.

Well, you're not injecting "liquid", so-to-speak. Once the refrigerant leaves the expansion valve, it becomes a saturate, and thusly superheated. It's purpose is to assure that return gas to the compressor does not exceed the manufacturer's maximum recommendation, as such the TEV is cooling the return gas. It is harmful for a compressor to operate "unloaded" with high gas temperature (and low mass flow) returning to the compressor.

I don't argue that an accumulator can be a good thing. I just wonder what it is that we are attempting to compensate for by adding it to anything. It is not really a protection device like a circuit breaker in an electrical circuit.

In a particular scenario that I encountered with a low-temp 10hp application with a short lineset (the manufacturer did not provide accumulator on new condensing unit), when coming out of defrost, liquid would make it to the compressor, and result in oil pressure safety trip. After conversation with Copeland, it was determined this to tbe the cause. Initially we compensated for the lack of liquid protection by increasing superheat setting, which of course de-rates capacity somewhat). However, on an unrelated service of the equipment, we took an opportunity to install an accumulator, and re-adjust the TEV to normally accepted settings.

P.S... the 7-1/2 horse CU that just took a dump on me (and broke a crank) did not have an accumulator!

Domestic heat pumps have an accumulator, for example, that is sized sufficiently to hold the entire charge. I imagine there is a good reason for it, although I am not sure what that reason is.

I would propose that the short linest between the evaporator (outdoor coil) and the compressor in the heat mode would produce the same symptoms as I described in my 10hp freezer example. Furthermore, in the heat pump application, I invision the outdoor coil (in heat) to ice up and unload readily in very cold and humid conditions amplifying such occurences.... not to mention defrosting affects.

The size of an accumulator is based upon the size of the error,

I don't know about calling it "error". It is a matter of understanding the characteristics of the operation of the equipment / machine, and applying the controls and devices to ensure dependability and perform the task in which it is commissioned.

[Dan]

In a particular scenario that I encountered with a low-temp 10hp application with a short lineset (the manufacturer did not provide accumulator on new condensing unit), when coming out of defrost, liquid would make it to the compressor, and result in oil pressure safety trip. After conversation with Copeland, it was determined this to tbe the cause. Initially we compensated for the lack of liquid protection by increasing superheat setting, which of course de-rates capacity somewhat). However, on an unrelated service of the equipment, we took an opportunity to install an accumulator, and re-adjust the TEV to normally accepted settings.

You are stating my case, Herefishy. Adjusting the TEV to compensate for a defrost event makes no sense, first of all.

Second, why are we assuming or accepting liquid return after defrosting?

I think "error" is still applicable. Although you do have a silver tongue:

I don't know about calling it "error". It is a matter of understanding the characteristics of the operation of the equipment / machine, and applying the controls and devices to ensure dependability and perform the task in which it is commissioned.

[Gary]

An accumulator adds very little superheat. Just insulate it.

[frank]

Spot on Herefishy.

On a heat pump air conditioner the defrost is nearly (if not always) carried out by the hot gas method, i.e. the reversing valve is energised to flood the outdoor coil with "hot gas". As this coil will be 90% full of liquid it will be pushed down the suction line and has to be collected by the accumulator to protect the compressor - hence the accumulator must be able to hold the entire charge.

Frank

[condenseddave]

Originally posted by Gary
An accumulator adds very little superheat. Just insulate it.

I agree. I've tested this on several occaisions on low temp equipment, for the sake of my own curiosity, and it's almost unnoticeable.

That said, I've been taught, as Dan has, that the accumulator should be sized for the entire charge. As I'm certain Dan can verify, this almost never the case on parallel racks that have accumulators. Some conventional CUs do have large accumulators, however, the difference in superheat is still almost nil.

If the accumulator were to actually load to full capacity with liquid, I'm afraid that you're in deep trouble, anyway.

Setting the superheat to compensate for defrost problems is poor practice, also. The reference to a stuck open hot gas bypass check is a good one to show how badly flooding can and does occur in some instances. It is not a pretty sight.

[Andy]

Hi,
Suction accumulators are of no earthly good without a boil off coil fitted in the bottom of them. This is usually fed by warm liquid from the main liquid line, or occasionally by hot gas.
Regards. Andy