The Homemade Heat Pump Manifesto

[Vlad]

Originally Posted by BradC

In the case of a fixed expansion device (orifice or cap tube) I agree with you completely.

A TXV or properly controlled EEV will open up further to maintain superheat thus increasing the mass flow in the system in line with the reduced condensing temperature. You move more heat from the evaporator. If you don't have the heat to move, then your SST drops until the system reaches an equilibrium.

I just want to remind that TXV is not a capacity control device. Its function is to maintain superheat and protect compressor from liquid return if you think that TXV will take care of everything this will never happen.

Originally Posted by BradC

Additionally, as the liquid entering the MD is cooler it has greater enthalpy and therefore you increase the evaporator capacity on a mass flow basis.

Subcooling liquid refrigerant is beneficial, but we are talking about sensible heat which is nothing compare to latent heat (this is where your full capacity and BTUs are hidden). Also you can subcool refrigerant without losing head pressure by adding liquid line/ suction line HX

Originally Posted by BradC

My point is as a DIY'er, if you are aiming for super COP's it's something you should really consider. Now if you are heating water for a hydronic application, then oversizing the condenser is going to buy you nothing, but if you are trying to cool a space (like I am) it's like printing free money.

As DIYer especially in such complicated field like refrigeration (remember "magic in the pipe") you have to be reasonable. Aiming record high COP's using low knowledge(in most cases guys have no special training) and scrap parts is not realistic. If you get COP in 3.5-5 range it is perfect for the time and money you spend.

I would suggest go step by step using as much original design as possible. When you get more experience go step up.

I have enough knowledge to understand most of the processes happening "in the pipe" but sometimes I get really frustrated because in real life we have too many variables and circumstances then in theory.

[AC_Hacker]

Originally Posted by Ko_deZ

Sorry for the wait. I have only had an hour today...
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WARNING: NOT TESTED IN REAL LIFE!

This is great!

I happen to have an Arduino laying about, so I'll check this out to see if it works on this side of the planet.

I have the lm35 part, I also have 1-wire temp parts. Out of curiosity, why not use 1-wire temperature probes? There are 1-wire libraries available, and no additional parts are required? But either will work fine.

-AC_Hacker

[Ko_deZ]

I have never used the 1 wire temperature sensors, so that is the main reason for not choosing those. Yeah, and the fact that I had all of these things around.

If you wish to change to 1 wire sensors, then that should be manageable to do. Have you tried it yourself?

Btw, attached a image on how to connect the thermal sensors.

[BradC]

Originally Posted by Vlad

So, by removing MD you have compression ratio = 0, then your power input will be 0 and efficiency will be infinite.

I've avoided answering this until I'd thought about it and settled on a polite answer.

The polite answer is: "bollocks!" If you remove the metering device (MD) you are simply pumping a liquid and are subject to the losses of the pump and the liquid friction. To state that the "power input would be 0 and efficiency would be infinite" shows a distinct lack of understanding of the laws of physics.

Refrigeration is limited by the laws of physics. Nothing more, nothing less. It's not "magic in the pipe". If you see a phenomenon you don't understand, it's not because it's magic. It's because you don't understand the the physics behind it.

Don't make refrigeration out to be something mythical. It's very simple and can be very easily explained by the laws of physics. When you see something you don't expect, it's because you have not considered the cause. Instead of just saying "Oh, it did that and I don't understand", read and learn _why_ it did that and figure it out. It's not magic, honest!

As your evaporating temp and condensing temp converge, the losses the in the system reduce to a point where your losses become those of fluid friction (which is relatively complex) and things like the friction of the piston rings in the cylinder and the resistive losses in the stator (which is relatively simple). If you could make your Te == Tc then yes, your cycle would be very efficient, but then you'd do far better just moving a fluid around and using big HX (Like _oh_ chilled water).

I have a 7KW fan coil in my roof. It's rated at 7KW at a Td of 10K. Nothing magic about that. With an evaporation temperature 10K less than the incoming air temperature and a flow of 1000M3/h it'll take 7KW of heat out of the air. Make the Td 15k and it suddenly allows me to pull much more out of the air. It's not magic, it's physics.

Telling people to expect and be happy with a COP of between 3.6 and 5 is belittling and condescending. I don't have a degree in refrigeration. In fact if it hadn't been for this particular thread I'd have never picked it up, but I've had a load of "experienced" people telling me it can't be done.

Do the calculations. The physics stack up. Just because you haven't seen it done does not mean it can't be done if the calculations and theory is correct.

Read, experiment and learn. It's the way of the future. Don't ask. Figure it out for yourself and then ask to see if anyone can pick a hole in your theorem.

If you don't reach for the stars, you'll never hit the moon. Honestly, if I can do it _anyone_ can.

[AC_Hacker]

Using a vapor-compression machine to move heat from one place to another is very interesting to me because it leverages the principles of low-exergy heating & cooling. Finding ways to reduce still further the exergy of an already low-exergy process is a challenging and excellent endeavor.

From the standpoint of low-exergy heating & cooling, the idea of a greatly over-sized condenser is very interesting to me, if the surface area of a HX is greatly increased, the temperature of the working fluid doesn't have to be as high (lower exergy) for a given amount of heat to be moved, so efficiency should follow.

* * *

I have been enjoying the previous exchange, but only to the extent that it improves my understanding of the principles behind vapor-compression machines, and how to increase their efficiency.

So, from that point of view, the exchange has been only partially useful. Don't get me wrong, hot air is a great thing, but HVAC equipment seems to make much better use of it.

I think that the proof is in the pudding, and BradC has a major batch of pudding he is cooking up and I, for one, am very interested to see how it turns out.

-AC_Hacker

[Geo NR Gee]

Originally Posted by AC_Hacker

I think that the proof is in the pudding, and BradC has a major batch of pudding he is cooking up and I, for one, am very interested to see how it turns out.

-AC_Hacker

I am sure a lot of us are interested too!

[BradC]

Originally Posted by AC_Hacker

From the standpoint of low-exergy heating & cooling, the idea of a greatly over-sized condenser is very interesting to me, if the surface area of a HX is greatly increased, the temperature of the working fluid doesn't have to be as high (lower exergy) for a given amount of heat to be moved, so efficiency should follow.

This is a vital point that can't be emphasized enough.

Heat exchange is the process of transferring energy. This is a very simple case of thermal conductivity. The greater the differential between the fluids (air is a fluid too) the more heat is conducted per unit of contact area. Conversely, the lower the temperature differential the larger the required area.

Design and engineering is always a compromise. My 7KW air cooled condenser has a designed "condenser split" (the difference between the ambient air temp and the condensing temp) of 20K! That means on a 40 degree day it condenses at 60 Degrees (and incidentally the compressor draws about 12 amps). It could be made more efficient by using a condenser with a 10K split, but the manufacturer made the decision to limit the physical size of the condenser. It would be more efficient, but it would be twice the size and more expensive.

Look at the gradual increase in size mini-split units have gone through as mandated efficiency ratings increase. The compressors are not getting any bigger, but the condensers are.

They are being *designed* to be more efficient.

Another point I want to raise with regard to efficiency is VSD control over compressors. Everyone talks about using the VSD to reduce the compressor speed to save power in times of low load. Copeland rate their scroll compressors to 70Hz. Now that's not incredibly significant for you Americans, but for us in 50Hz land that's headroom for an additional 40% capacity. That means we can start out with smaller compressors, reducing all frictional losses and therefore increasing efficiency. We don't need to size for the worst case, we can size for an average load and ramp up when we really need it. It's a shame three phase compressors are not as easy to find as single phase. Maybe someone needs to design and build an affordable single phase VSD for PSC compressor motors.

Just because we hack stuff together from scrap and parts does not mean we have to be content with as-bought performance and efficiency. We have the technology and knowledge to make it better because we are not shacked by the compromises manufacturers must make when they design a product.

I'll say it again. If I can do this stuff, literally anyone can. Refrigeration is not black magic. It is governed by simple (and not so simple) physical phenomenons. Having said that, you don't *need* to understand the cycle in depth. I study it because it fascinates me, but you can get better than average results by following simple guides and rules of thumb.

[Vlad]

BradC
I will tell you the story: 2 years ago I wanted to get geothermal heating in my house. I knew it was almost impossible for DIY. But I am stubborn and never give up (sometimes give in for a period of time... ) I built a drilling machine from my head and it drills like crazy. I missed the point that to drill the hole is not everything, but just beginning. Grouting is the evil that wrecked the whole project. I tried many things to push that darn grout. Finally I came to the point where I can see the light at the end. I can drill 100-200 feet deep but only can grout MAX 50 feet. This is how theory becomes practice.

This is just an example how people overestimate things and only follow their idea but the real trouble is not there. I tried many things in my life (and keep trying) but looking back sometimes I think "I was so naive and why I didn't listen to that person...."

"Magic in the pipe" is an exaggeration but there is reality we all live in. This reality dictates how and why. Manufactures are not stupid they ran thousands of tests to determine the best REASONABLE performance. This "performance" is already built into components. Yes you can improve performance like many racing guys do with their vehicles by paying big $$$.

BTW I don't work as refrigeration mechanic for few years. The whole HVAC industry really sucks. I was really fascinated by refrigeration and it stays my hobby and will stay for many years. If you say about refrigeration it is all clear it means you didn't get it yet.

[Ko_deZ]

Vlad, I think you are mistaking customers to be properly intelligent and efficiency focused. That is not the case as any marketing person will tell you. A good design and good reputation of durability will sell a whole lot more that a small increase in efficiency. Do you know may people who are willing to have a in-door unit more than twice the size of a normal one?

What BradC is saying is the truth. The larger and more efficient heat exchangers will _always_ give you a better result as long as you apply the same knowledge on that system as you do on one with smaller heat exchangers, that being pipe sizes, gas pressures and all of that. Of course, if you start pulling in liquid in your compressor, then you are in trouble, but you would have the same problem with a smaller exchanger doing the same thing there. You can actually never have too large heat exchangers if your goal is high efficiency. The lower delta T, the more efficient, that is the one and only truth to that equation. In addition, larger size reduces the air-speed need, so you get a more quiet and more efficient fan system as well, improving the system even more. Given a _huge_ HX, like a full floor, you would not need a fan at all, although that gives you other problems you must handle of course.

[AC_Hacker]

Originally Posted by Vlad

I built a drilling machine from my head and it drills like crazy. I missed the point that to drill the hole is not everything, but just beginning. Grouting is the evil that wrecked the whole project. I tried many things to push that darn grout. Finally I came to the point where I can see the light at the end. I can drill 100-200 feet deep but only can grout MAX 50 feet.

Vlad,

I was very interested in how that part of your project played out.

So, I assume that you put in a series of 50 foot deep holes, right?

How many holed did you end up drilling?

So when you tried to do the grouting, exactly what did you try?

Did you try MIX-111?

Did you try various water-to-grout ratios to solve the problems?

Did you try some kind of tube ('tremmie tube') that you could pump grout through? If you did, what diameter(s) did you try?

If you did try to pump grout, what did you try to use as a grout pump?

As you surely know, I decided not to grout, since my holes did not pass between aquifers... so I didn't have the grouting adventures you had.

-AC_Hacker