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Old 12-20-13, 04:26 PM   #11
jeff5may
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In the parallel arrangement, the units can still be balanced against each other to equalize refrigerant flow. They will tend to frost up simultaneously; as the more efficient evap frosts up, it builds up back pressure due to its expansion valve closing.This increases the flow to the clear evap, which will then frost up until it develops back pressure... Process repeats until both are clogged enough to call for defrost. Upon cycle reverse, both fill up with warm gas and the colder one "sucks" more gas until both are free of ice.

Also with series-connected units, there is no real way to balance heat transfer. Due to the single path through both units, superheat control may not be feasible. One unit will always beat the other out of its share of the load. In a water exchanger with counterflow, this effect can be minimized, but in a crossflow air exchanger with equal intake temps, it's nearly impossible.


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Old 12-20-13, 07:01 PM   #12
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Quote:
Originally Posted by jeff5may View Post
...With series-connected units, the flow is constant in both units, and with the added surface area, this flow will increase. The increased flow...
What is your rationale for, "and with the added surface area, this flow will increase"?

How is this different from doubling the length of a pipe with a fluid flowing in it?

Doubling the length of the pipe will double the surface area, but doubling the length also doubles the friction, so to maintain flow rate you will need higher compressor power.

Did I miss seeing that you included "increasing compressor power"?

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Old 12-20-13, 07:51 PM   #13
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AC,

In the series-connected circuit, I agree that there is more friction present. With a cap-tube metered evaporator, the flow will actually decrease due to this added frction and pressure drop, and the cap tube will need to be modified to adequately feed the larger evaporator. If not modified, the increase in suction superheat and compression ratio could literally burn out the compressor.

A TXV or EXV metered system will attempt to do this automatically if the sensor is moved to the end of the second evaporator. But with the same size compressor, the added heat transfer area will bring the heat gained in the evaporator to the point where the added pressure drop will literally be "sucked out" by the compressor (by increased mass flow and resulting expansion at a higher leaving temperature).

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Old 12-21-13, 03:10 AM   #14
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I'm impressed with the knowledge and reasoning - and it all seemed so simple - I'm glad I asked the question now.

I'm going to do a bit of research on the two machines to see if it feasible to put them in parallel rather than series - maybe I'll have to invest in a couple more txv's and junk the capillaries as I'm certain they will be mismatched.

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Old 12-21-13, 05:55 PM   #15
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Have done both series and parallel.

In parallel used separate TXV (cheap on ebay or surplus), totally different evaporators, one conventional air, the other water tube in tube. If cold enough to freeze up the air coil, the water coil handled the entire load (air txv basically shuts off). Like others have said, you do need to use 2 TXV with sense bulb a foot or more ahead of the junction on the suction line.

Have also put a 4T air evap on a 2T compressor, which is basically what you are doing in adding a series evap. TXV is so much more efficient than cap tube that you should use TXV for series connected evaps also, but only one evap needed.

Another advantage of the series connection is that the coil will not freeze up until air temp is about 35F if evap overall large enough..

Other part is to oversize the condenser, that way part of the condenser works as a supercooler. Only disadvantage there is that output air (or water) temp is lower; but the lower temp means higher efficiendy.
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Old 12-21-13, 06:17 PM   #16
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Something to beware of is oil return. R290 (either as a refrigerant or an additive) easily solves the oil return issue, but connecting the evaporators in series makes it unlikely to be an issue in the first place.

I have used an ejector approach where I divide up the evaporator into two sections. One part goes where an evaporator would go on a normal system. Between the first evaporator and the TXV, there's an assembly known as an ejector (a kind of pump that has no moving parts apart from the fluid itself) which basically creates a reduced pressure zone for the second evaporator. I also have a phase separator between the first evaporator and ejector tube to supply liquid to the second evaporator. I used a way oversized TXV (5 ton in a 1/2 ton system, can be found cheap on the surplus market) to control flow to the second evaporator. (The reason why the second TXV must be way oversized is because the pressure difference across it is much smaller than in normal use. While a R22 TXV normally sees on the order of 100-200 PSI across it, the TXV for the second evaporator would probably only see 10-20 PSI or so.)

BTW, the ejector approach was first used in the Prius A/C system. By cooling the air in two stages, dehumidification is improved. I highly doubt it would offer much (if any) advantage for a heat pump, and it would certainly be difficult to design such that it would work both ways. Just stick to a series connection.
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Old 12-21-13, 06:58 PM   #17
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Quote:
Originally Posted by mejunkhound View Post
...In parallel used separate TXV (cheap on ebay or surplus), totally different evaporators, one conventional air, the other water tube in tube. If cold enough to freeze up the air coil, the water coil handled the entire load (air txv basically shuts off)...
mejunkhound,

Very interesting approach.

So, are you saying that you ran air-to-refrigerant AND water-to-refrigerant concurrently in parallel?

Did you notice any "hunting" as has been suggested is the behavior of non-identical evaporators in a previous post?

Also, are you saying the air-to-refrigerant side shut down when it frosted up?

Was the water-to-refrigerant being drawn on the whole time?

Reason I'm interested is that I'm cooking up a hybrid ASHP/GSHP and I plan to run the ASHP most of the time but when the temp takes a nosedive, I'll bring up the GSHP side.

From the description you gave, it might be possible to allow the system to make the changes... to be self-regulating, depending on frost conditions.

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Old 12-22-13, 01:40 PM   #18
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by the question:

.. you ran air-to-refrigerant AND water-to-refrigerant concurrently in parallel?
Yes

Did you notice any "hunting" as has been suggested is the behavior of non-identical evaporators in a previous post?
Previous post mentioned 'hunting' for a system without 2 separate TXV, I had 2 separate TXV

Also, are you saying the air-to-refrigerant side shut down when it frosted up?
Yes, since that evap could boil off less refrigerant, the txv on that evap closed down

Was the water-to-refrigerant being drawn on the whole time?
This was a system I built over 30 years ago, but recall I had a tsat that turned on the water when air tem was 50F or below.

Now, having said that, the system I have now operates with 2 separate compressors, which provides redundancy in case of one system failure.
The compressor failed when I was out of town, and DW had a hard time keeping warm, so figured to have 2 separate systems.

What I have now is I replaced the 4t compressor in my air-air HP with a 2T compressor. COP increased from about 3 to 4.5 or so at 40F.
Built the 5T GSHP described in you epic thread as a stand alone system. Above 40F the 2T air-air operates and never freezes with the oversize evap as the coil temp only goes down to 32.1F. The 2T air-air is able to heat the 5300 sq ft house OK at 40F outside.

At 40F outdoor temp, and outdoor thermostat switches over to the GSHP by way of a few custom relay and FET circuits. The air-air HP blower operates for both systems, but at different speeds.

It is all also intertied with the fireplace, which has water wall pipes and 2 old car air conditioner condensers in the ductwork and a hot water circulating pump. When there is a hot enough fire, the HPs are locked out.

May have time later in the week to copy the control schematic and post in a separate thread.
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Old 01-21-14, 07:17 PM   #19
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Quote:
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Now I have my defrost circuit monitoring the temperature on both sides of my evaporator I got to thinking, would a bigger evaporator be more efficient?

Heat pumps obtain their energy from low grade heat (the atmosphere) and 'convert' it into high grade heat using a compressor. The warmer the temperature of the gas entering the compressor the less hard the compressor has to work to compress the gas and raise its temperature.

Whilst monitoring my evaporator I have seen variations of temperature difference of between 1C and 6C depending on the ambient temperature. The exit temperature is invariably less than ambient temperature (it approaches it at very low temperatures).

So, my thought is, with a larger evaporator there would be extra surface area (and time) for the refrigerant to absorb heat from the atmosphere, hence improving the efficiency of the heat pump.

Has anyone tried this? The argument seems sound but does it work in practice?

I have a machine with a dead compressor but a perfectly serviceable fan and evaporator and I'm tempted to try it out but would love to hear any feedback on my proposed hack before getting out the cutters and brazing torch.

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Hi; The laws of physics and thermodynamics have a basic rule regarding heat transfer or movement. From hot to cold and directly related to the difference of temperatures from one medium to another and inversely to the insulation value of the medium. Aluminum and copper transfer heat rapidly while wood fiber glass of course insulate' Larger coils on either end of the cycle means more efficiency. Note that most late model ac units outside units are huge.
I don't know enough to tell you how to balance the internal workings of this switch. I notice that my unit has 4 banks of coils with a txv to each while old units had basically 1 metering device to the coil. Owen
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Old 01-21-14, 10:51 PM   #20
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Maybe you could build a radiant wall on the exterior of the house. Something similar to those homemade hot water solar collectors. Run a heavy glycol solution through there, have that run through a big HX to act as a giant evaporator. Maybe also combine design of thermosyphon heater to encourage passive air flow across the radiant panels. Make them modular so you can add on more over time.

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