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Old 06-09-17, 07:25 AM   #1
Sccoupe
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Default Any trick to sizing a coaxial condensor?

---After getting a couple of geothermal holes drilled, I'd like to start testing/experimenting with some actual heat transfer. My initial thoughts were to replace the condenser of a window unit with a coaxial condenser and pump the water though while measuring temps. The question is, If I start with 12000btu window unit, do I just replace the air condenser with a 1 ton coaxial condenser? Is it this simple?
---Then I start thinking why only set this up for summer? Why not pickup a 1 ton portable heat pump to use year round? What gets me here is that they are 12000btu in cooling mode and 10000btu in heating mode. Does this still mean to use a 1 ton coaxial condenser?

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Old 06-09-17, 08:05 AM   #2
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https://www.ebay.com/itm/251555614530
https://www.ebay.com/itm/191796804772

I'm not an expert on this yet.. But my understanding is that you should over size it slightly.. So if you have a 12k unit I would go for the 1.5 ton coil. But I would also look into flat plate heat exchanges. Those can be had cheaper in some cases. As long as you have a clean water supply and from the sounds of it you will have a closed look system with bore holes..

I'm sure the others will chime in shortly
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Old 06-09-17, 08:13 AM   #3
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The second link that you posted is the exact condenser that I have. So it sound like perhaps an 8000 or 10000 btu unit is what I should be aiming for. Is there any oil in an condenser that I need to be concerned with replacing after the condenser swap and recharge? I'll be storing the old coolant for the recharge, but not sure about how much oil is usually in the condenser and how to know if a unit needs more added.
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Old 06-10-17, 07:09 AM   #4
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Here's your performance data:
https://packless.com/drawings/coil/performance/Classic

For as cheap as you paid for the one coil, you could buy another and gain maybe another 10 - 20 percent COP, and still spend less than the retail price on a new HX! With the operating parameters you posted, this coax coil should do as well as a prefab GSHP. Remember, though, that the HX must work against a decent DeltaT to transfer lots of heat. If there is not much difference of potential, the exchanger won't be very effective. More water flow is not always better. The literature shows the relationship precisely for your HX.

As far as oil, I wouldn't worry too much about it. If you didn't just poke a big hole in the circuit and spew foamy froth everywhere, most of the oil will still be in the compressor. Window units don't contain much extra oil in them, as the refrigeration circuit is small. The only oil that will leave the old unit is whatever tiny amount sticks to the walls of the old HX you remove.
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Old 06-10-17, 03:03 PM   #5
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I got the coaxial HX installed on an 8000btu window unit. Tested with a small 1gpm pump and a 5 gallon bucket of water and it heated the bucket to 125F in no time.
Waiting for a socket fusing unit to arrive monday ($39 shipped on ebay. Search pipe welding, not socket fusion) and then can get some pipes dropped into ground. Two 90 feet deep holes are done and water comes up to within 15 feet of the surface in both holes. Hopefully this will be enough to let the 8000btu AC help out this summer and further testing. The holes will need grouted with something, but havent figured what i'll do there yet.
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Old 06-12-17, 12:17 PM   #6
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Quote:
Originally Posted by Sccoupe View Post
---What gets me here is that they are 12000btu in cooling mode and 10000btu in heating mode. Does this still mean to use a 1 ton coaxial condenser?
If it were me, I would go with a Brazed Plate HX. Not so expensive, very efficient, small, and the engineering data is abundant. Brazed plates work great in a closed-loop system that you have.

The 12K cooling/10k heating is a written confession of inefficiency.

Heating is always more efficient than cooling, because the heat energy of the compressor is folded into the output, whereas in cooling, the unit needs to rid itself of the compressor heat to deliver cooling.

Says to me that the HXs were grossly undersized to provide optimum heating.

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Old 06-30-17, 12:31 PM   #7
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As I learn more about this stuff it just brings up more questions.

So, lets say I select an extra large water HX for a window unit. It does well to get rid of the heat into the ground loop. Now we add a reversing valve and our extra large condenser becomes an extra large evaporator, correct? Does this not mess up the super heat that took so much time to get right in the A/C operation?
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Old 06-30-17, 09:58 PM   #8
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Nope. In the reverse cycle, your metering device will be regulating the superheat the same way as before. If you're using a capillary tube, it will allow the same flow in the reverse direction. I myself love TXV metering devices. However, if you employ tx valves, there needs to be one for each heat exchanger. These either have internal check valves or need check valves plumbed across them, to bypass the txv for the heat exchanger being used as a condenser.



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Old 07-01-17, 05:45 AM   #9
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Ok, this makes sense now. None of the heat pump google diagrams that i've seen so far show two metering devices. However a search for two txv's brought up some info on heat pumps having an indoor metering device and an outdoor metering device with check valves for each. Some it seems use a TXV outside and a cap tube on the inside.

With two metering devices, it doesnt seem that having exactly 'matching' HX's is all that important though one still wouldnt want a 5 ton outdoor HX with a 2 ton indoor HX.

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Old 07-01-17, 12:48 PM   #10
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It all depends on the air or water flow and the temperature gradient driving the heat transfer through the exchanger. With a water loop or well outside, the ground temperature and pump size pretty much dictate the minimum size for that HX. Indoors, the main function of the unit will dictate what size and type of HX to employ. For example, if your main goal is hydronic slab heating, a larger plate HX and circulator pump will extract many more Watts of heat at a lower condenser temperature than if you're making hot tap water. The hundred Watts or so of pump power used to circulate slab water (versus a DX coil in a hot water tank) is usually well offset by the increased COP of the refrigerant cycle.

With an air source HX, the outdoor unit should have an expansion valve in it when it is being employed as an evaporator. This allows the unit to draw much more heat in when outdoor air temperature is higher. A cap tube reacts to changing conditions much more slowly. Whenever outside temperature drops below freezing, the TXV can get you the extra heat gradient needed to drive heat flow. A cap tube is optimized for a very narrow set of conditions. Outside of these conditions, the cap tube system is leaving potential efficiency on the table.

However, adding expansion valves and check valves adds complexity to the refrigerant cycle. There are sensors and moving parts included that a cap tube rig simply doesn't have. When things go wrong, a cap tube rig is much easier to troubleshoot. With a small capacity system, the energy saving potential is not as significant, so most manufacturers use cap tubes to minimize cost and complexity. When the real world conditions stray outside the unit's design window, the units simply shut down.

On the other end of the spectrum, lots of newer units employ electronic expansion valves. The refrigerant cycle becomes less complex, as the EEV can control flow in both directions, so there is no need for extra mechanical components. The EEV serves the same function as a TXV, but it is electronically controlled. Instead of a pressurized sensor bulb balancing the opening and closing of the valve against spring pressure, an EEV relies on a digital controller. The digital controller connects to temperature and/or pressure sensors, interprets the observed conditions, and adjusts the EEV accordingly. As you may suspect, the controller can be programmed to behave however you want it to.

In many modern heat pump units, the programming of the controller is the main (and sometimes only) cause for differences between units. This is increasingly prevalent in mini-split and inverter-compressor driven units. Many ecorenovators have purchased different size mini-split units of the same brand, and found little to no difference between the outdoor units. Some of them even had the same part numbers on key components, or identical outdoor units.


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