New Zealand DIY Water Source HP.

[geoheatnz]

Hi Jeff ,
both of these plates are freebies , so im just doing what i can with them.
The evap is a 10t or 35kw , so more than 100% over capacity. If i used the dual plate it may be 200% over and i was concerned with gas velocity and oil return to compressor. I think it best to use dual plate as the condenser then i have plenty of room at bottom of plate for subcool and a reservoir for liquid that wont raise the high side pressure.

The water is heating the underfloor slab so i do not need high temperature water just a gradual warm up. The closer the high and low side are the greater the COP.

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Having too low high side pressure is actually a problem, as higher head pressure (to a certain extent) ensures that your refrigerant condenses faster and at higher heat flow. Especially with refrigerant-to-water, where the refrigerant may need to work extra hard to force a temperature rise in the water.
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If you have the surface area on your condenser the condensing temp will get close to your water out temp , the rate of transfer will slow as the temperatures approach. Its better for the compressor capacity but not for the plate heat transfer , really just economics if you are designing and buying the plates. No good aiming for near infinity.

[jeff5may]

No, here's what will happen in your scenario. On a cold start up, the massive condensing action will gobble up your high side pressure. The compressor simply cannot move enough gas to saturate the condenser. Due to the fact that expansion valves are a pressure driven device, the mass flow through the one in your unit will be puny until the condenser builds pressure. Thermostatic or electronic, it doesn't matter. They are designed for a maximum amount of flow at a certain (high) pressure. Below that pressure, they sit wide open, passing however much flow that pressure and/or superheat control will allow.

Heat pumps all do this, and modern units have a built-in (indoor fan) delay to help speed the initial pressure-temperature ramp up. This one small feature is a big time selling point for mini-split manufacturers. With a huge condenser, your unit will do the opposite, slowly ramping up output until useful heat is produced. The effect is not trivial. Every heating cycle, you will be wasting x amount of electrical energy just to get started.

My point is this: So what if your compressor is only using 80% energy on startup if it takes ten minutes to build up momentum? Yes, it all depends on cycle time and duty cycle and etc., but a quick start-up will make up for a whole lot of "a couple degrees closer approach". That ultra-super-efficient heat exchanger could keep your desuperheater from working at all.

[stevehull]

These last two posts are superb and I look forward to the dialog between these two experts. By "thanking" both, I am not taking any side, but am very intreagued with the subtle choices that have made this NZ water source heat pump so efficient. But as Jeff asserts, further increases in efficiency are there - so this is a discussion you should follow.

I certainly am learning a lot!


Steve

[geoheatnz]

If the system is correctly charged the unit will begin to produce heat almost immediately , i can understand that a split needs to get up in temp so there is not a cold breeze blowing but this doesnt mean the unit would not condense at a much lower pressure if allowed to , it is just for comfort of the occupantants. The condensing temp is a factor of surface area and fan speed. (if we assume a constant refrigerant flow).
With the underfloor i do not high temperatures so as soon as the system starts , heat is flowing and this will gradually build up as floor warms.

Take some ball park fiqures , If at start up my water return temp to my condenser is 28c then my condensing pressure will be 32c to 33c , at this point in time my COP will be the highest as cond temp is lowest and ground loop will be warmest , as the hp runs over the hours COP will drop as conditions change.

I admit that it is all theory at the moment , but only using 50% of the condenser plate gives me pretty close to the same size plates for evap to cond.

[jeff5may]

Yep, I understand where you're going here. This delves into the theoretical and practical world of mechanical engineering. The main difference here is that in the design phase of most stuff like this, the entire whole thing is modeled on some kind of computer. The engineer(s) then throw curve balls at the unit to try to avoid building a turd. They can do these things easily and quickly because they have expensive simulation programs that rely on formulas and specs that have been reviewed and historically proven in reality. Once the thing satisfies the design team on paper (or on screen), a preliminary build is made and tested rigorously. And then and then and then... many revisions later, the design is finalized.

The main reason for all of the trial and error in the design process is because there always exists a difference between "ideal" and "real". What your theory suggests and what you build for real never equal out completely. If you have very strong faith in theoretical simulations, spreadsheets, and rules of thumb, prepare to be let down when the rubber hits the road. Even with superconducting supercomputers and decades of engineering experience at your disposal, be prepared for Murphy's law and mother nature to wreak havoc on your idea. Happens every day in industry.

Back to your project.

If I'm being too critical of your plans, let me know and I'll back off. From your past postings, I can tell this project is real (somewhat rare on the internet) and that you have the skill and ambition to make this thing as good as it can be (even less common). As always, you can take or leave any of my suggestions for whatever value you may or may not see. Doesn't hurt my feelings a bit.

You said the smaller heat exchanger is rated for more capacity than the compressor you plan on using, yes? With an extra heat exchanger plumbed in series as a desuperheater, your recent post of having 30-35 degree condensing temp doesn't make much sense. Unless your DHW tank is merely "warm", it is going to be sitting there in the 50+ range. If it possibly can, the refrigerant flowing through the desuperheater will rob heat from your hot water tank to feed the floor. If your hot water tank is solar powered, no problem...if it's not, hmm...I can't call it from here.

For the sake of reality, let's say your DHW tank is going to top out at 60 degC, and that you plan on maintaining 20 to 25 degC room temperature. If you have a hyper-efficient slab floor with closely spaced piping, you can get away with 30-35 degC return water temperature. This will keep your system from lagging too badly. This puts your supply water temp up into the 40's already. Assuming a 10 degC gradient in the indoor condenser puts you in the 50's on saturation temp and pressure. So topping out in the low to mid 60's on your high-side PT curve may very well happen, even with your oversized condenser and a really efficient slab.

On the outdoor side of the system is where you have a hard boundary: the freezing point of water. The fact that you have a flowing stream makes a gargantuan refrigerant heat exchanger very valuable. It will be very difficult for your outdoor heat collector to freeze the flowing water, due to the enormous latent heat involved. If you design it right, the underwater heat exchanger will have a hard lower limit as the thing starts trying to freeze the stream. With a large surface area in the plate heat exchanger, this reduces the refrigerant-to-water delta T, and your hard boundary shows up when it is most needed: the coldest days of the year.

What I'm getting at here is it really pays off to maximize your heat gathering ability. The compressor can only move what gas it is fed. A couple of extra degrees on the suction side makes more difference when the unit is running hard. This happens every cycle as well with a hydronic system without some sort of heat store tank. I understand your ideal conditions, but if you want to maximize your savings, what you really need to design for are those really cold days. Then when ideal conditions present themselves, the system will perform better than expected.

[geoheatnz]

Hi Jeff ,
i have no concerns about discussing design and dont have hurt feelings or anything:) .
I fully understand the effects of "over condensing" and have seen the effects on air cooled refrigeration systems several times.
Im sure if i could of afforded it , i would of had SWEP plates designed and ordered them , probably costing $1500 -$2000 nz dollars.

My house can be heated with a return temp of 30c , going on previous history with the air to water hp . Depending on the out door temp the flow was 5k higher. The condenser then was a 15kw unit and approach was 11c so condensing about 46c . I expect these to be reduced with the new evap and condenser but on the same compressor and same water flow .
The refrigerant im using is r407f which has a high discharge temp compared to r22 , the desuperheater pump will only come on when discharge temp over 60c .
ok got to go to work.

[geoheatnz]

Here is a quote on how eevs work from Carel.

Over the last decade electronic expansion valves and controllers have taken a substantial part of the expansion valve market due to the increased energy efficiencies they provide. The EX valve does not rely on a minimum pressure drop across the valve, as a motor drives the valve open and closed. This enables the condensing temperature/pressure to be floated down with the ambient temperature which saves energy. As much as 25% energy savings can be achieved by using an EX valve and floating the condensing temperature/pressure. EX valves require an electronic driver to operate them and are usually available as a kit to make selection simple. Usually a driver kit and an EX valve is all that is required.

[geoheatnz]

I assembled all refrigeration side today and have attached photos. Just reversable water side to complete . Only burnt my self once .

[jeff5may]

How do you plan to control the electronic expansion valve in this system? I'm satisfied with using TX valves in the units I assemble. I'm sure I am not the only one who is eager to figure out how these things actually perform. The OEM's speak of these devices like they are secret weapons.

Here's an article that is the opposite of what most manufacturer claims are like:
http://www.achrnews.com/articles/950...ves-the-basics
And the follow up that is intentionally vague:
http://www.achrnews.com/articles/942...pansion-valves

If​ you look a little deeper into the design side, the valves are indeed rated just like TX valve as far as selection. They are rated just like the mix and match valve bodies and power heads. What this tells me is that a certain part is designed for the range and refrigerant it will be metering. So yes, you can force a smaller Delta p, but then the capacity of the valve is reduced. Dad blame it, I'm talking like an engineer again! Sorry about that.

[geoheatnz]

I am using the complete Carel system.

Expansion valve driver

E2V fully welded

These use a pressure transducer and temp sensor at outlet of evap.