multiple evaporators

[billthomas]

I am building a small freezer with 2 evaporators driven by one compressor. The metering devices are cap. tubes. Has anyone had success with this? All I have experienced so far is one plate has excess frost back, while the other plate stays warm and dry.

[herefishy]

Cap tube? Dual evaporator? Doesn't sound like a good idea to me. I would suggest that you be sure that the lineset is equivalent length and pressure drop in and out of the evaps (if you're going to continue this arrangement). Also, I would suggest that the Tee's be arranged so the suction and liquid lines dead-end and tee out in opposite directions to ensure equal pressure drop in and out of the evaporators.

Have you considered up-sizing the cap tube, and feed the evaporators in series?

:)

[Preston Roy Powell]

Cap Tubes with multple evaps never work well you have trouble feeding.
1 how big is the freezer.
it can be done with one evap with expansion valve system

[billthomas]

I got it to work. Both evaps. fully frosted with a box temp of 3 degrees. (3 cu. ft. box with 1 inch of insulation). Balanced the mass flow to the cap tubes and evrything seems good so far. Yes, there are deadheaded tees. I am trying to get away from the expansion valves due to space restrictions and the type of evaporators.

[billthomas]

I haven't tried to increase the cap sizes. I have always felt that one evap would freeze and only send vapor to the next evap in series, but have never tried it. Maybe I finally will.

[herefishy]

If you do feed the evaps in series, just adjust your refrigerant charge to reach an outlet temperature of the downstream evap to near the saturation temperature of the refrigerant (at near design box temp). That will tell you that you are feeding all the the evaporator(s) circuit. :)

[billthomas]

If I feed them in series, can I use the same size cap for both evaps?

[frank]

I don't see how you can connect them in series and expect to get equal duty out of each evaporator.

Any heat absorbed into the first evaporator is going to boil the refrigerant in the first evaporator which is then going to discharge into the second evaporator! Flash gas and liquid in one evaporator and pure liquid in the first evaporator does not equal same duties from both evaporators.

The only way you can consider equal duties from 2 evaporators served from the same refrigerant circuit is to connect them in parallel with equal lengths of liquid line and eqaul heat loads.

Any thing else and the loads are out of balance.

[herefishy]

frank, if the (suction line) temperature is at saturation at the outlet of the downstream evaporator, what would that tell you? (If) you charge the system to near saturation at the outlet of the downstream evaporator, you (inherently) WILL NOT be feeding only vapor to the second evap.

Mind you, the (2) evaporators are cooling the same space.


Any heat absorbed into the first evaporator is going to boil the refrigerant in the first evaporator which is then going to discharge into the second evaporator!

....Not if the system is properly charged. In a properly sized system, the (first) evaporator will have only enough rate of heat transfer to boil off (about) one-half of the liquid (in it). The liquid that is still boiling will continue on to the second evaporator.

Think about it as one (longer) evaporator.

:)

billthomas...... No, you would employ a cap tube of essentially double the flow rate of the cap tube(s) that you are using to feed each evaporator, now. You will need to calculate the cap tube size for the series application.

Technically, you may be able to use the same diameter, but a shorter lenght...... but that may not be true, in general. Sizing the cap tube for the duty of both evaps combined would be necessary

[frank]

Herefishy


"The liquid that is still boiling will continue on to the second evaporator".

That is exactly my point. Even the first evaporator will be absorbing heat from within the space so it is bound to boil the refrigerant. It is this boiling refrigerant that will be feeding the second evaporator - not pure liquid.

If both of the evaporators are in the same space then, yes, combined the system will be in balance but you won't get equal duty from each evaporator.

[herefishy]

It is this boiling refrigerant that will be feeding the second evaporator - not pure liquid.

frank, "Pure liquid" does not emit from the expansion device. The refrigerant begins "boiling" the moment it enters the low pressure side of the system due to it's temperature being above that of the saturation temperature of the refrigerant at that (low) pressure. If the evaporator was "pure liquid", we would then be referring to it as a "LIQUID LINE" and not an evaporator if no evaporation is ocurring !! :)

Just for instance: If one pound of any refrigerant had an enthalpy of 10 btu's per pound, the expansion device was metering 1 pound per hour, and each of the two evaporators were rated at 5btu's per hour, the first evaporator would not have enough heat transfer to evaporate all of the 1 pound per hour of refrigerant. :)

In order for an "evaporator" to function properly, the refrigerant must have boiling (i.e. evaporating) refrigerant throughout it's circuit. "pure" liquid is NOT evaporating! furthermore, pure liquid would not exist in the low pressure side of a properly operating system. :)

[twcpipes]

BillThomas, unless this is an exercise to see if you can parallel evaporators, could you not change the distributor and feed both coils with the proper size, same length cap tubes? assuming, of course, both coils are of equal tonnage.

[herefishy]

twc.... that's what he has already done, and it is working fine. frank and I are now in a debate regarding a suggestion I made for piping the evaps in series. :)

[frank]

Herefishy

Assume that the 2 evaps are in the same box - any heat absorbed in the first one will boil into the second one therefore the second evap in series will contain more flashgas than the first one and will then contain less liquid than the first.

As it is primarily liquid that absorbs the heat the 2 evaporators will be un balanced.

The box will obviously be rated at the total of the 2 evaps.

How would you charge such a system? on frostback? - it would be a different matter if the 2 evaps were interconnected with a small length of large diameter pipe but quite a different story as it is intended to connect between the evaps with capillary - or did I miss something?:rolleyes:

[herefishy]

frank,

Yes, you would connect the outlet of the upstream evaporator to the inlet of the downstream evaporator via the line size of the outlet tube of the evaporator.

YOU WOULD NOT BE CONNECTING THE EVAPORATORS WITH A CAP TUBE !!!!!

ONLY the inlet of the upstream evaporator would be fed with a (larger) cap tube.

The two evaporators would essentially be one. Only one cap tube would be used used.

billthomas, were you clear on that idea?

Gee, Im sorry. It never would have ocurred to me that someone would visualize a cap tube between the two evaporators.

sheeeeesh!!!!! :)

[frank]

Way to go mate!

[twcpipes]

with the two evaporators connected with a single pipe create a problem with homogenous mixture in the downstream coil?
Just curious.

[herefishy]

Just for instance: If one pound of any refrigerant had an enthalpy of 10 btu's per pound, the expansion device was metering 1 pound per hour, and each of the two evaporators were rated at 5btu's per hour, the first evaporator would not have enough heat transfer to evaporate all of the 1 pound per hour of refrigerant

[billthomas]

herefishy yes, I understood the single cap. idea. Thanks

[RogGoetsch]

My two cents?

In a series application, the first evaporator would have a larger percent of its surface in contact with liquid refrigerant than would the second, so given same size, etc., would presumably do more work.

Schaefer made an ice-cream box (horizontal) with two evaporators in parallel, cap-tube type. One evaporator is overhead, equipped with a drain tray and defrost heaters. The second is actually imbedded in the wall of the box, 3/8" tubing on 3" centers. (I know because I had to do emergency surgery on one when an employee accidently shot it when he was playing with his gun! Hit the tubing dead on! (Yes, the store is in a very bad neighborhood!))

The in-wall evaporator is one very long tube which serpentines along and acts very much like Herefishy's series evaps would. That is: on initial pulldown, the critical charge of refrigerant is not enough to chill more than the first few feet. But as the wall comes down in temp, the frost line moves right around until the entire perimeter is frosted evenly.

One nice feature is that you can look at any cold wall horiz ice cream box and see at a glance if there is a low charge. If the wall is evenly frosted, it ain't. (Unless, of course, the second cap-tube is plugged and all the charge is in the wall, or it isn't totally frosted but the fan motor is out or, or....)

[herefishy]

well rog.... that's just the nature of a cap tube system. If you applied an expansion valve into the scenario, you would have better refrigerant control throughout the evaporator.

So what's the difference in surface area for an evap that is rated for the entire load, and two evaps that are rated for one-half of the load (each)??? No difference.


Just for instance: If one pound of any refrigerant had an enthalpy of 10 btu's per pound, the expansion device was metering 1 pound per hour, and each of the two evaporators were rated at 5btu's per hour, the first evaporator would not have enough heat transfer to evaporate all of the 1 pound per hour of refrigerant

[Abe]

2 evaporators, same space
Ofcourse the first would boil off the refrigerant and absorb heat etc before passing on to the next stage, common sense tells me that and i agree with frank. That is assuming its connected in series

Me thinks.........why dont you just have one big evaporator?

[RogGoetsch]

Originally posted by herefishy

So what's the difference in surface area for an evap that is rated for the entire load, and two evaps that are rated for one-half of the load (each)??? No difference.



I was responding to the earlier comment regarding the evaps cooling unevenly. The point I wanted to make is that if it matters for the design, you would have to take into consideration the fact that the first evap would do as high as 100% of the cooling during initial pull-down, with the load shared more evenly as the unit reached design temperature. That might influence a decision to feed parallel or series.

Anybody who's tried to repair a Schaefer case with one plugged cap would vote for series any day!

Schaefer's dual cap tube, parallel evap design was used not because it mattered for cooling, but for ease of fabrication and defrost considerations, I assume.

[herefishy]

If you used one evaporator for the application, then you would have to assume that only PART of the evaporator is doing 100% of the work during pulldown, then, right?

But it is the refrigerant that is doing the work...... not the evaporator. the evaporator represents ONLY the medium by which the transfer of heat is conducted.

[Dan]

Nice discussion, all. Being a supermarket guy, I picture a single evaporator spread the length of a 12-foot dairy case or reach in. If the cap tube is undersized, then this single coil will starve.

But it will starve from front to back or bottom to top. The refrigerated space will suffer equally, because it likely won't starve left to right.

If there were a left and a right evaporator piped in series and things were not right regarding the expansion device, then you would see uneven temperatures left to right.

Beyond that, I really don't have much to add.:)

[herefishy]

During pulldown (in a cap tube system), the reason that the evaporator "starves" is due to the fact that the evaporator capacity increases (proportionally) to the increase in TD.

For instance, an evaporator rated at 10btuh at a 10degF TD (SST=-10degF, design box temp 0degF), has a rating of 1btuh per 1degF TD. So if the box is at 40 degF and your SST is 20degF, the evaporator has a capacity of 20btuh.

As such, if the cap tube is rated for the design application of 10btuh @ -10degF SST, yes the evaporator may seem to be "starved" due to the increase in evaporator capacity during pulldown, because the cap tube is sized to feed the refrigerant at a (lesser) rate for the design temp of 0degF box at a capacity of 10btuh.

But actually you are doing MORE work (during pulldown) than when you see the coils "evenly frosted" at design conditions.

[toprunner]

Two evaporators with capillary tubes as expansion device and one compressor can and have been done. You need a switching valve and some other stuff to control the refrigerant flow (charge) since it will merge to the evaporator with lowest heat load (lower pressure). I can promise it is not easy to do.. but as I told.. The problem has a patented solution even for two compartments with different temperature level !

[The MG Pony]

Wow You win the Necrothread award mate!

Any case, might as well chirp in on this 5 year old thread, I agree with here fishy, to evaps joined in series is just a bigger 1 evap, it obeys all the laws a single big one would.

Two separate independent ones how ever will not when connected parallel or a dual temp set up serially.