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View Full Version : Homemade 900W waterchiller, are these measurements ok?



Henry.
20-11-2010, 12:40 AM
Have just built a prototype compressor unit waterchiller to handle 900W load and are now up and running what seem to be quite ok, well the water at least get about the wanted temperature anyway.

Although some measurements feels like they are a little to high and would be very happy for some help in this because im using only a capillary tube for this nothing else since the load will be quite constant but i dont know how to fine tune these systems if needed.


The water out from the heat exchanger needs to be under 20C so i just guesstimated the evap needed to be around -10C based on the heatexchanger size and how it its built internally, so thats what the captube is based on and so far so good. According to Dancap which i used for this i needed a captube that was 1.4 mm inner and 2.06 meter long.

The only captube i had in the whole house was 1 mm that was 3.96 meter long but paralleling two 1 mm would be equal to one tube that was 1.41 mm which was fine with me and is what i have on the system now but i dont know if its to long or to short for an optimal system with this load and so on.

So here are some measurements on what i think you guys might need. This is when the system had been running for only about 35 min since the temp in the room started to rise to much (small room) for my own comfort and the water started to be to cold (13C since it will starting to freeze up in the exchanger at about this temp) so i shut it off but minutes before so did i took these measurements:


Condensor Air
In: ~26C
Out: 37.3C

Condensor temperatures
In: 86C (this feels waaay to high?)
Out: 40.2C

Evaporator/Heatexchanger
Casing: ~ -2C (some distance from captube inlet)
Outlet: 13C (~10cm from evap but this tube between the comp and evap is very short only about 20cm (8") if that might matter

Pressure
Highside right after condenser: 13.8bar (once again feels high?)
Suction: 2.5bar

Gas used
BBQ propane (only gas i can get hold on, i use a big filter/drier though)

Compressor
SC15D HST/HBP


If any other info needed just give it a shout.
 

 
The captube i use is the ONLY captube i have and i cant get any similar one, or any other at all for that sake, so i dont dare touch it until i really really know so guesses about the captube length or so arent really what i need due to this unless its a good qualified one.

The pressure in the system when its at room temp (21C) is 7.8 bar and that to feels like its to much gas in it but i need to have this much to get the watertemp i needed. The evap obviously isnt filled enough to act fully as it could but due to the amount of gas now as for before so will the watertemp even be lower then needed after some time. That it self is not a problem in one way because the comp will later on shut down with a controller when the right water temp has been reached.

Before i got more gas in the system the water temp out was only 20 – 18.8C which was little too high but the pressure was lower with about what i feel more normal 11 - 12 Bar high and around 1.5 - 2 Bar low if i recall right. When i put in more gas the pressure went of course up to the pressures stated before and the evap was at about the same temp. When i then put more gas in it the water now went down to 13 C although it took several minutes but down to 18C happened much faster now and it kept going.


As for the water temp the system is ok but im worried about some measurement since im not that into captube designs. So should i mess with the captube and/or lower the pressure a little or do anything else or perhaps do nothing?

The evap needs quite a lot to be filled since its quite big internally but this of course means a higher pressure and thus higher highside pressure which cant be good over a certain threshold which i dont know so i dont know whats ok and not. I could leave it but i dont like these high temperatures and i want the system to live a good life by many reasons.

Any help would be highly appreciated about if these measurements are ok or not so i can go forward with this.

Gary
20-11-2010, 02:49 AM
None of this seems excessively high. On the contrary, the load seems light.

In the heat exchanger, is the flow counterflow, cross flow or parallel flow?

What are the entering and leaving water temperatures?

Henry.
20-11-2010, 10:19 PM
Good to hear that it might even be underloaded, was just a little concerned about the high temp on the discharge but it might actually be ok anyway then, thats good to know.

The heat exchanger is actually a tube oil cooler built with many small tubes inside where the coolant then will go up and down between them due to 4 baffles inside it so i dont know what to say about flow direction its almost all of what you mention. I got it for only 7$ on a flea market so i use it for this since those real flat ones cost some but its total capacity is more then enough for 900W according to info i found on the net.

Although i havent connected it as it really should i have connected it in total reverse so the inlet and outlet of chilled water/oil have instead been the evap compressor side and the water that are supposed to chill the oil/water is now the water that are been chilled. I did this because it was a lot easier to connect it all to the comp system due to the gas flow but i didnt thought it should do much to the cooling efficiency.

Right now i testing the system with a 900W heating element and small pump in a full 1.5 liter bottle but i cant really directly measure the entering temperature to heatexchanger since its not open anywhere between to put a temp sensor. But if i feel on the tube so would i guess that its only a very few degree over the leaving temperature it feels pretty much the same temp of course i isnt. It just dont have any time to heat up all the water in the bottle.

The leaving temperature depends totally on how long i let the system go it could go down to 10C and past this but the water will start to freeze in the evap at this point so i shut it off before but it takes around 8 - 10 min for it to get to 10C.

The pump is a small submersible 3l/min pump, the flow needed would be 2l/min but a guess is that its pumping only 1/4 of this now or so now due to the way i have put the water hoses which are quite high up if this might matter in anyway for the whole.

 
I have just put more gas in it to see if it would be any difference and it will go down much faster from at least room temp then before but the more gas i put in the pressure of course rises quite much so i stopped at ~15Bar.

I just now tested to let all the circulated water (about 2 liters incl. exchanger) to heat up to 42C what i believe would be little over the real temp but most likely more around 35C or so, and then started the system.

It took about 5 min for the leaving water from exchanger to go down to around 20C and about 6 min to get it down to 17C where is about where i want it to be or little lower but it is still going down but it takes some more minutes to get for example to 15C where i normally shut it off.

If i do the same when the system has been off for a while and the water temp is at room temp (this time 26C) so did it take 1 min 20 sec to get down to 17C.

Gary
20-11-2010, 10:33 PM
Least efficient would be parallel flow, where the two fluids travel side by side in the same direction. Ideally, the fluids should flow in opposite directions (counterflow) or crossflow.

Gary
20-11-2010, 10:35 PM
The pump is a small submersible 3l/min pump, the flow needed would be 2l/min but a guess is that its pumping only 1/4 of this now or so now due to the way i have put the water hoses which are quite high up if this might matter in anyway for the whole.


The flow matters very much. It changes everything.

Get the required pump flow. Run the system until the water is at the desired temperature. Then report all of the temperatures and pressures as you did in your first post, so we can see what it is doing.

Henry.
22-11-2010, 01:08 AM
Ok i see now that i might should have written that different. The flow i mention isnt what is required for the heat exchanger or so but is what will be needed to the thing all this will be connected to later on.

But i have now checked the flow instead of guessing and its was actually quite closer then i thought it would be it was about 1.6l/min. I have now rearranged the hoses and i now have pretty much 2l/min trough the exchanger which will be the flow later on needed.

I have put more gas in it yesterday because when i tested it before it took too long time for it to go down under 18C but now its much faster almost like i want it.

But i started it and let the leaving water temperature from the exchanger go down to 15C where about i would like it to be and these temperatures are from exactly that point:


Condenser Air
In: ~25C
Out: 37.8C

Condenser temperatures
In: 66C (slowly climbing)
Out: 41C (btw the captube is not coiled around the suction, but will be later on)

Evaporator/Heatexchanger
Casing: ~ 0C (~2 inch from captube inlet)
Outlet: 13C (~3 inches from evap, going down slow)

Pressure
Right after condenser: 15bar
Suction: 2.9bar

Heat exchanger water temp
Entering: 15.2C
Leaving: 15C

Gary
22-11-2010, 05:36 AM
It needs more refrigerant.

Using an R22 (similar pressures to propane) pressure/temperature chart, the saturation temp is about -7C. The evap outlet temp is 13C minus -7C = 20K temp difference (20K superheat). When the water temp is 15C, the superheat should be about 5K.

With the water temp at 15C, add propane slowly until the evap outlet temp is just slightly above 0C, then show us all of the new temp and pressure readings.

Henry.
23-11-2010, 04:11 AM
To add propane at the time its 15C until the suction will go down to 0 is almost impossible because when it hit 15C it will still go down so i cant add propane until the evap outlet hits 0 at 15C watertemp.

But i just took the temperatures and so on at a point where i had about 14.2C and from there it went down much slower then from 20C to 15C and this after i had put quit a lot of propane in the system several times. This when letting the system cool of several minutes beween next time i added more gas in it but it never got down to 0 at this point and the pressure started to go upp quite much but the readings are from about this point.


Condenser Air
In: ~25C
Out: 39.4C

Condenser temperatures
In: 59C
Out: 41.4C

Evaporator/Heatexchanger
Casing: 6C
Outlet: 4.2C

Pressure
After condenser: 18.6bar
Suction: 4bar

Gary
23-11-2010, 05:57 AM
Is the bottom of the compressor cold?

If the bottom of the compressor is cold, slowly remove refrigerant until it is lukewarm.

Henry.
24-11-2010, 12:11 AM
I felt underneath the suction on compressor when i filled it before so it didnt got overfilled and it was cold almost all the time but i just figured it had to do that it was because i had just filled it with quite a lot of propane that had chilled it.

But when i felt on the bottom just when i started it now so yes it became colder then room temp right away so it was quite overfilled.

These results are when it started to go a little warm at the bottom after i emptied quite a lot of propane from it but i did this under several minutes. The water temp wont go any lower then about 14C now.

And i just discovered that the inlet water temp probe to heatexchanger hade been connected wrong so it actually measured the outlet water, the same as the other probe already did. So now its a more normal difference in inlet and outlet water temp with about 7 degrees when this was changed.

Oh well here are the measurements when comp now started to be a little warm after quite some propane had been emptied but it could have been ok even with a little more propane left in it:


Condenser Air
In: ~25C
Out: 35.3C

Condenser temperatures
In: 67.3C
Out: 40C

Evaporator/Heatexchanger
Casing: -4C
Outlet: 11.3C

Pressure
After condenser: 13bar
Suction: 2.4bar

Gary
24-11-2010, 12:32 AM
Hmmm... now we've let too much out. Let's add propane until the high side pressure is about 17 bar. That should be pretty close.

Normally, this would not be this difficult, but I don't have a chart for R290.

Henry.
24-11-2010, 01:07 AM
Ok i just let it run some time to get stable temps and pressures so i didnt cared about water temp this time:


Condenser Air
In: ~25C
Out: 39.3C

Condenser temperatures
In: 68.5C
Out: 41.3C

Evaporator
Casing: 3C
Outlet: 1.3C

Pressure
After condenser: 17bar (when finished writing it all up it was down to about 16.7 but the temps where pretty much the same)
Suction: 3.4bar

Gary
24-11-2010, 01:38 AM
I would estimate saturated suction temp at about -3C. So... 1.3C minus -3C = 4.3K superheat, which is just about right.

The saturated condensing temp would be about 47C. So... 47C minus 25C = 22K TD, which indicates a very heavy load.

This gives us a pretty good balance between SH and TD, although the load is heavy.

If the high side pressure drops a little as the target water temp is approached, say closer to 16 bar, that would be ideal.

I'm thinking we could remove just a little refrigerant, bringing the high side pressure down to 16 bar and still have a reasonable superheat.

Henry.
24-11-2010, 03:08 AM
Ok a tiny bit gas was removed until it were very close to 16 bar at 15C water temp but the system had been on before this and were a little heated but it had been off some few minutes before i got this results so its quite alright i guess.

This is at more or less exactly the point when the water got to 15C:


Condenser Air
In: ~25C
Out: 38.6C

Condenser temperatures
In: 66.2C
Out: 41.4C

Evaporator
Casing: 2C
Outlet: 11.8C

Pressure
After condenser: 16bar
Suction: 3.4bar


And very close to when the water got to 15C the pressure actually went down very very little to pretty much exactly 16 bar and after the water got little colder it went up very little.

Btw under the comp suction on the casing its actually a little cold but not more then 2 - 3 inches underneath it and it not cold at all under the comp but i guess this is just fine?

You say the load is heavy you just mean that its a heavy load generally? I would be quite suprised if the captube length happen to be exactly whats needed for the load.

Gary
24-11-2010, 04:08 AM
The cap tube could be a little more restrictive. It is running heavily loaded as a result, but not excessively so. You might try slowing the water flow a little, until the evap outlet temp drops back down to about 5C.

Henry.
30-11-2010, 11:49 PM
Little more restrictive could be equivalent i think to make it a little bit longer but i only have what i have so its would not be possible so i have to restrict the water flow then as you say.

I just realized that the airflow true the condenser is at the wrong direction as i see it. As it is now the air that is sucked in will first pass the side where the hottest tubes are and exit at the coolest tubes. That would heat the cold tubes up as i see and that is no good since the condenser would not be able to work at peak performance then, dammit more work to do.

If this is the case so would the outlet from condenser be higher then it should and that would affect the measurements i have mentioned.

But i have the system going quite fine now and i could lower the flow a little and see as long as i get the outlet at about 15C so would it work just fine.


Thanks a lot for your help in this Gary it was very informative and helpful and highly appreciated!

Regards Henry

Gary
01-12-2010, 02:46 AM
The airflow direction through the condenser could make a significant difference. After you reverse this, let us see the new numbers.

Henry.
01-12-2010, 05:22 AM
Yes as i thought, but it would need some major rebuild to change this but i will get back as soon as this is done.

Henry.
03-12-2010, 10:35 PM
Ok that made a noticeable difference. This is when the watertemp out from evap were around 15C where i wanted it but still going down and it took almost 4 min for it to go there but after it went down to 15C it went down much faster from there.

Ok new results with airflow at right direction:


Condenser Air
In: ~25C
Out: 36.5C

Condenser temperatures
In: 68.6C
Out: 35.5C

Evaporator/Heatexchanger
Casing: 7C
Outlet: 10.6C

Pressure
After condenser: 14.3bar
Suction: 3.1bar

Gary
04-12-2010, 03:18 AM
What changes did you make? Just airflow direction?

Henry.
04-12-2010, 05:18 AM
Thats the only change i did, although quite some gas leaked out when i changed a hose since the valve didnt close at time as it should but i put in about what i guessed was the ammount that had leaked out.

Gary
04-12-2010, 06:17 AM
It all looks pretty good now.

Henry.
04-12-2010, 08:58 AM
I see, not even a little more or little less gas perhaps or so? Would be amazing if its all good now.

nike123
04-12-2010, 10:13 AM
I see, not even a little more or little less gas perhaps or so? Would be amazing if its all good now.

For Propane:
14,3 barg is 45°C
3,1 barg is -4,5°C

If I read correctly your temperatures:
You have superheat of 10,6--4,5=15,1K
You have subcooling of 45-35,5=9,5K

I would say that you have capillary which is slightly restricted (too long for present load) and therefore somewhat higher superheat and subcooling.
But I am not that good in balancing capillary systems to be able to say with certainty what you should do next.
Maybe to remove some refrigerant until subcooling is ideal (8,5K by Gary, if I recollect correctly) and than see superheat.
If evaporator water in temperature is 15°C and condenser air in temperature is 25°C, ideal evaporator superheat should be around 5K or 6K.
I would love to see water in and water out temperatures and flow rate of evaporator in your last data. If you have flow of 2l/m than water in-water out should be 6,45K for 900W load.

Your discharge temperature and discharge superheat (23,6 K) is not too high.

Wait for more answers from more experienced colleagues with capillary tube systems.;)

Edit:
Your evaporation temperature is too low for water entering evaporator at 15°C. It should be around 4-5°C. That is result of high evaporator superheat. That mean lower evaporator efficiency and high possibility off evaporator freezing and damage.
I think that now is good time to work on capillary tube length (and superheat).
And, by what I read so far, your condenser is too small for present load (approach 45-36,5=8,5K is little high and dt 36,5-25=11,5K is really high, which mean not enough surface or dirty and not enough air flow).

Gary
04-12-2010, 08:47 PM
Nike, where did you find a P/T chart for propane? I was unable to find one, so I have been using an R22 chart.

Gary
04-12-2010, 08:50 PM
I see, not even a little more or little less gas perhaps or so? Would be amazing if its all good now.

I wouldn't say it's all good, but it may be the best we can get it without a major re-design.

Gary
04-12-2010, 09:09 PM
Your evaporation temperature is too low for water entering evaporator at 15°C. It should be around 4-5°C. That is result of high evaporator superheat. That mean lower evaporator efficiency and high possibility off evaporator freezing and damage.


The evap temp was low even when we had low superheat.

We could try reversing the water flow through the evap in case it is parallel flow, but the OP assures us that it is not.

nike123
04-12-2010, 10:19 PM
Nike, where did you find a P/T chart for propane? I was unable to find one, so I have been using an R22 chart.
Cool-pack software has it in refrigerant calculator module.
http://i53.tinypic.com/2d0z86e.jpg

nike123
04-12-2010, 10:30 PM
The evap temp was low even when we had low superheat.

We could try reversing the water flow through the evap in case it is parallel flow, but the OP assures us that it is not.


Ok, But I would be concerned with negative evaporation temperature in water/refrigerant evaporator, or I would consider using brine.

Henry.
04-12-2010, 11:28 PM
Thanks once again Gary!
Thanks for the equations nike its really not my good side..

The evap temp should by the way be corrected down. The sensor didnt seem to touch the casing as it should, dont know how long it has been so but not more then a few days at most since i think something got stuck in the wire to it when i changed a hose to the system yesterday.

Although i now dont really know exactly where i should put the evap sensor to get accurate readings. Should it be low to the bottom opposite side from captube inlet (as it is now), several inches up from the the bottom or same side as the captube inlet and close to it?

The temp on the evap needs to be quite exact as i see it so it wont affect the whole by wrong temperatures so we would know that it all work as it should.

And one of the hoses gasket wasnt tightened as they should so some gas have leaked out during night since i changed one yesterday but i put in what i thought were lost based on the pressure when it was running later.

So here are new results to go on just in case and with the evap sensor moved slightly and fastened tight but not sure the gas amount is right at this point and it feels like a little more should go in. This is when the water out from evap had a temp of 14.3C which is fine for me:


Condenser Air
In: ~25C
Out: ~35.6C (Measured this quite late after the water temp went down so take with a pinch of salt)

Condenser temperatures
In: 65.3C
Out: 34.3C

Evaporator/Heatexchanger
Casing: -3C
Outlet: 9.9C

Pressure
After condenser: 13.8bar
Suction: 3bar



Water temp evap
In: 20C
Out 15C

At very close to 2l/min and there is about 2 liter of water in the system.


I have calculated the captube so the evap would be at few degrees below freezing point at 900W so it would be chilled down quicker. And yes i will have antifreeze in it later on if needed but it doest freeze until the water out would go down to about 8C which is lower then i would need.


Gary, i have a pdf ptchart with all ref gases in existance but cant upload it because its over 100kb which is top but here is a one for only R290 if needed:

Gary
05-12-2010, 01:44 AM
The evap temp we are discussing is the suction pressure converted to saturation temperature on the P/T chart.

Since you have a P/T chart for propane, how about reporting the saturation temps along with the pressures, so we don't have to look it up?

Have you tried reversing the direction of the water flow through the evap?

Henry.
05-12-2010, 02:42 AM
Aha sure no problem now that i know this, only a little sad it wasnt mentioned before, but this will be done from no on.