The Homemade Heat Pump Manifesto

[BradC]

Originally Posted by randen

But if I understand what Vlad was speaking of its heating we are after and not the cooling side. We need to optimize the heating capacities of the unit and what occurs on the evaporation side is less of a concern as long as its picking up enough heat.

He's pretty right, although if you are talking about maximising heat then you want the evaporation temperature as high as you can practically get it.

If you look at a spec sheet for a medium temp fridge compressor, they specify capacity usually at 3 or 4 evaporation temperatures. As you would expect, the higher the evaporation temperature (or saturated suction temperature) the higher the capacity of the unit. Its a simple matter of mass flow through the pump. The denser the input, the more it can move per stroke. This is countered by the higher the output pressure, the less efficient the pump becomes as it has pumping losses to work against.

To maximise your heat *moved*, you want to evaporate as high as you can and condense as low as you can. More volume through the compressor. Although by increasing your condensation temperature you make the compressor work harder, generating more heat, and you also allow your condenser to reject more heat into the environment (as you increase the Td). You could keep doing that until you burst a pipe, burn a valve or toast the compressor. Better to make the condenser more efficient (or bigger).

If you really want to make a better heater, swap the R290 out for R22. It generates more heat while being compressed. I seem to recall something to do with the density and the shape of the molecules causing more friction during compression, but don't have a reference to hand.

[randen]

Brad C & AC_hacker
Today spent a lot of time lowering the charge and adjusting the pressures. Slowly increasing the charge during the day. As it happens suction of 75 psi and head 300 psi are the pressures providing ample heat. The shop is slowly warming. It went from 12 DegC to 18 DegC The office floor is going along for the ride as well. Its at 18 DegC as well. The current draw is 11amps. If the compressor has to run 24/7 to keep the shop warm thats about $1000.00 for the season. Hopefully I can shut it down for a few hours as I believe its producing a little more heat than with the cap tube. Well its better than Oil.

Thanks for the though Brad C on the R22 that had run accross my mind to. I was thinking the industry is moving to the higher pressure R410 must be for some reason and maybe this is why. This higher pressure R410 may allow for a larger window of heating cooling capability. During the summer air-conditioning season will I need to adjust the TXV or can I leave it be at those pressures??

I think my next move is more insulation and I have an entrance door thats a little leaky. The last part of the day was spent on the controls. The control system will take input from two thermostats one for the office floor and the other for the shop air handler. The relays will operate the zone valves and start the heat-pump and circ.pumps.

Randen

[BradC]

Originally Posted by randen

I was thinking the industry is moving to the higher pressure R410 must be for some reason and maybe this is why.

I'm quite cynical on this subject. My thoughts on 410a are more along the lines of patented substances and Dupont and Honeywell lobbying.

On a more practical note, 410a has a higher heat capacity than 22 which means you can use smaller compressors, but more importantly smaller lines. Less volume means less frictional losses, so in theory its more efficient compared to its predecessor.

I still feel the rest of the world will eventually migrate to HC refrigerants, but the industry in the US will maintain HFC as the status qou to maintain regulation over refrigerants and keep Dupont happy.

R600a owns refrigerators. Europeans are doing all sorts of cool experimental stuff with split systems and R290, it's only a matter of time.

[jeff5may]

Ok, so I revisited the articles to gain clarity. I was thinking like BradC and things just didn't make sense. More suction pressure = more mass flow = more capacity, right? Hmmm...

Then I read this:

"...the high solubility of mineral oils in propane could represent a problem, especially for applications such as heat pumps displaying high suction pressures... the experimental results reported in the present work can therefore be explained by means of the strong influence that the propane solubility could have on the sealing effect of the lubricant between piston and cylinder... the solubility is shown to strongly decrease by increasing the superheating; as a consequence, the viscosity of the oil strongly increases with superheating until reaching a maximum at around 30-40K superheating... the suction superheat would have a positive influence on the volumetric efficiency."

and from the other article:

"Refrigeration capacity and COP differs only slightly from R22
data. However, the suction superheat with R290 has essentially
a greater influence (see Fig. 4). In other words: R290
profits in capacity and efficiency from useful superheat, the
use of a heat exchanger between the suction and liquid lines
is therefore an advantage.
R290 has mainly favourable thermodynamic properties and
low energy requirements during compression. Pressure levels
and volumetric refrigeration capacity values are very similar
to R22, however, there is a large difference in enthalpy,
density, mass flow and isentropic compression exponent
(operating temperatures)...

R290 has an extraordinary high solubility with conventional
lubricants and Ester oils. This characteristic is of course
desirable for the oil circulation in the system. However, it
can lead to a considerable decrease of the oil viscosity in
the compressor especially at low oil temperature and high
suction pressure. In addition to this, there is a strong degassing
effect in the crankcase and lubricating spaces
which is, amongst other things, due to the enormous volume
change with the evaporation of R290. This leads to
high oil carry over (foaming), reduced performance and
stronger wear on the moving parts (also see para. 3.2)...

Expansion valves should be specifically designed for R290.

The use of R22 valves is of course possible but at higher
evaporating temperatures it can lead to insufficient superheat
(different pressure / temperature relationship). A corrected
superheat setting would then be necessary.

Attention!
The minimum discharge gas temperature should be at
least 20 K (try to attain 30 K) over the condensing temperature.
(see information in para. 2.1)"

Then it hit me. R290 has MUCH more heat capacity than R22. How else could it transfer the same heat with only 40% of the mass? It also expands more than R22 when it evaporates. So in my case, running at 10-15 psi less suction pressure is actually giving the propane more time and dT to pick up heat in the evaporator. Meanwhile, my beer oil is now flat once it reaches the compressor. Both effects add to efficiency in their own way.

Since hardcore research into propane and butane refrigerants is just now being approved, there are a lot of blanks in the lanscape. But the wizards are all saying that the standard mass flow enthalpy density energy flux capacitor application models are going to be different for R290 systems than their current refrigeration law books dictate. What this means to me is that they will be hammering out new laws for propane in the not so distant future. But their crystal balls and cauldrons are beginning to show some images to interpret and conspire upon.

Meanwhile, ima hack.

[randen]

Would anyone Know what the head highpressure bypass valve in the compressor be set at?? How high could one safely run the compressor providing the current dosen't overlimit.

Randen

[Mikesolar]

Originally Posted by randen

Would anyone Know what the head highpressure bypass valve in the compressor be set at?? How high could one safely run the compressor providing the current dosen't overlimit.

Randen

Randen,
If your hermetic comp is the same as mine, it has a second port on the top that is caped off. It may be for a bypass but also MAY be used for a vapour injection (need to check this out). If so, head temps can be controlled with a solenoid and temp sensor.

Also, I didn't see a tank on the output of the system before the floor, but I didn't really look hard. You could control condenser flow rate better with it.

[AC_Hacker]

Originally Posted by randen

...Would anyone Know what the head highpressure bypass valve in the compressor be set at?? How high could one safely run the compressor providing the current dosen't overlimit...

You might try over at HVAC TALK... they aren't usually so friendly to DIY folks, but if you called yourself something like "tarheel" and misspelled a few words and made some grammar errors, they might help you. Especially if you said it was for a customer's beer cooler.

Good luck!

-AC

[Mikesolar]

Originally Posted by randen

Would anyone Know what the head highpressure bypass valve in the compressor be set at?? How high could one safely run the compressor providing the current dosen't overlimit.

Randen

I had read that 120C was at the temp activated the bypass but that the comp won't last long if the bypass is used regularly. It is different for different comps so we would have to look at the specific literature.

[Mikesolar]

Originally Posted by AC_Hacker

You might try over at HVAC TALK... they aren't usually so friendly to DIY folks, but if you called yourself something like "tarheel" and misspelled a few words and made some grammar errors, they might help you. Especially if you said it was for a customer's beer cooler.

Good luck!

-AC

I've been on there quite a bit in the last couple of years but unless you really want to take a beating, don't let yourself appear like a DIY guy. There are knowledgeable people on there but also a lot of hacks that talk like pros.

[MN Renovator]

They send an image of their EPA cert to the website to get marked as a professional member. If you don't you show up as a 'regular guest' or something like that.

I opened an account there and as long as you aren't saying you are doing something or asking questions that make it sound like you are installing or modifying equipment or handling gas or refrigerant they are receptive. The moment you ask for the limit of head pressure, they are going to ask for you to become on a pro on the forum and close the thread telling you to come back. You might want to do a search for udarrell He has a website with homeowners in mind and is on multiple sites where DIY guys are and even has a full website at Scroll to "MY AIR CONDITIONING PAGES using Ctrl+F where he talks about A/C stuff. On that same link go up about a page or two and there are other HVAC forum links that have more DIY friendly members. If you find guys like this they might give a crack at giving you an answer. There are lots of guys who really seem to know their stuff on Youtube, it takes some searching to find them but they are there. I even watched a video of a guy working on the same mid-80's furnace model that I have in my house and he said where the heat exchangers corrode out first and how to clean the burners, burner chamber, and other checks for my furnace. Very helpful but at the same time I know enough not to screw anything up.