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Old 12-20-13, 11:02 AM   #1
Acuario
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Default Evaporator sizing - is bigger better?

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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Old 12-20-13, 12:34 PM   #2
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Yes, a larger evaporator will make a heat pump more efficient. With more surface area for heat transfer, and the same compressor being used, the compressor will move more heat due to the lower compression ratio. How much more depends on your mass flow and lift.

The bugger here is matching the flows between the two units running off of one compressor. How easily that can be accomplished really depends on how the individual evaporators were designed. If they both have integrated distributors and txv metering, the job becomes much simpler. But if they don't employ the same metering method, or use cap tubes or electronic valves, they will hunt against each other.
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Old 12-20-13, 03:39 PM   #3
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But if they don't employ the same metering method, or use cap tubes or electronic valves, they will hunt against each other.
Are you saying, "they will" or are you saying, "they could"?

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Old 12-20-13, 12:36 PM   #4
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Acuario,

There is a point where making the evaporator too large and you loose a lot of efficiency. Take this to the extreme to make the point.

Imagine a standard 36000 BTU (3 ton) air conditioner with a standard evaporator. Now, put in place an evaporator the size of a house. The gas that fills the evaporator has expanded almost to room pressure and it has long given up all the latent BTUs. Yes, it is cooled slightly more than say an evaporator the size of a kitchen and only slightly more than an evaporator the size of a bed.

The point being that the pressure - BTU - cooling curves are not linear, but are S shaped. Evaporators work on the linear part of the sigmoid curve. Beyond that and the change in pressure vs temp curve goes rather flat.

Also, imagine the volume of refrigerant you would need!!

Does this make sense?

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Old 12-20-13, 01:59 PM   #5
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Yes, makes sense - house size evaporators are hereby scrapped :-)

What I was thinking of though was to link the output side of one evaporator to the input side of the other so connecting them in series - I didn't intend to use 2 x metering devices or running 2 evaporators in parallel, just more evaporator if you see what I'm thinking.

I would need to add charge to the system, both machines are about the same rating (around 5.5Kw) but that's ok.

So guys, worth a try ?

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Old 12-20-13, 02:34 PM   #6
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...so connecting them in series - I didn't intend to use 2 x metering devices or running 2 evaporators in parallel, just more evaporator if you see what I'm thinking...

As far as I can tell, most of the manufactured units are not optimized for maximum efficiency, but rather are optimized for maximum profit to the manufacturer, so there are certainly efficiency gains to be made by an experimenter.

I think that your idea is worth trying, but why you would shy away from running them in parallel, with duplicate metering devices? I see this done all the time in single evaporator units...



...so how would the principle be any different with two separate evaporators?

If you are seeking more heat from your evaporator, is there any chance that utilizing heat from the ground will work for you?

If your soil is easily drillable, there is a lot of heat to be had there.

Also, if you could direct sun-warmed air through your evaporator, it could be even more valuable to you than a bigger evaporator.

Best,

-AC
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Old 12-20-13, 02:09 PM   #7
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Why not run them in parallel? Wouldn't that give you the same surface area, but less flow restriction (not sure if that is an issue or not)?
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Old 12-20-13, 02:17 PM   #8
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I think Daox is onto something (evaporators in parallel). About 1/2 the resistance, a lot more surface area - and a bit more refrigerant.

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Old 12-20-13, 03:14 PM   #9
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I didn't intend running them in parallel as they are different manufacturers, different caps etc. so as Jeff says it may cause a problem with hunting with the mismatched metering devices - I don't have txv's to put in.

Ground heat is out where I live - dig down 10cm and you hit solid rock.

The evaporator is already positioned in a sun trap so I'm getting about as much boost as I'm going to get without moving the machine to one of the south facing walls - and I already have a machine mounted there used for heating the dhw.

The idea I had forming in my head was to bolt the units together, one on top of the other then feed the output at the top of one evap to the input at the bottom of the second. The top of this then feeds back down to the compressor. A fairly simple plumbing job.

I can wire the fans in parallel so they both run when the unit is on.

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Old 12-20-13, 04:10 PM   #10
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Both Daox and Stevehull make good points here concerning flow. Parallel flow is the way you want to go with the evaporator side. Assuming the same outdoor temperature at the two units, the flow will tend to be equally divided between the two units when plumbed in parallel. With this divided flow, the refrigerant will spend more time in each evaporator at a lower speed, and heat transfer will increase in both units simultaneously. With similar expansion devices in each unit, balancing the heat transfer is a simple matter. This is how the manufacturers optimize the heat transfer in multi-circuit exchangers like the one in AC's post. Defrost happens faster, but requires a sensor on each unit.

With series-connected units, the flow is constant in both units, and with the added surface area, this flow will increase. The increased flow causes pressure drops which add up to less effective drop in each unit, causing less delta-t in each at a higher flow rate. The heat transfer is still greater than with only one unit, but the units will tend to find equilibrium where one will transfer a lot more heat than the other. Usually, the first unit after the expansion valve will transfer most of the heat, and the other will be left with whatever is left. As temps approach freezing, the first unit will frost up and the load will be transferred by the second unit until it frosts up. Defrost cycles will be enormous.
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