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Re: AWHP superheat & sub-cooling
^ Fair-enough - good thought.
What I'll do is to have a set of ratings done by my condenser supplier, for various combinations of water entry temp & Tc,sat. From that data we should have a more clear idea of the system trade-offs.
I've already done this for a fixed Tc,sat & variable water inlet temp, but will extend the study to see where the sensitivities lie. Good one...
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During operation with a pump-around system, the water flow-rate setting will determine the number of water passes (times) through the condenser, & hence the dTw across the condenser.
If the condenser is sized to cope with the hot-condition such that the condensing area is sufficient for this, any additional over-surface can then be put to good use, without disrupting the condensing section.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
I haven't tested the Magoo Method (superheat 60-70% of TD), but it makes good sense. It should work very well over a wide range of conditions.
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Re: AWHP superheat & sub-cooling
Experimental feedback - AWHP test system - 30.07.09
Quote:
Evaporator:
Te,sat = 13'C (sat temp)
Te,sup = 20.8'C (vapour exit temp)
Ta,in = 24.9'C (air inlet temp)
va,in = 3.5 m/s (air inlet face velocity to evap face)
Ta,out = 21.5'C (air outlet temp)
va,out = 9.5 m/s (air outlet velocity - fan discharge)
Condenser:
Tc,sup = 64.3'C (superheated vapour inlet to condenser)
Tc,sat = 50'C (condenser sat temp)
Tc,sc = 43.7'C (liquid exit temp)
Tw,out = 39.8'C (water outlet temp)
Other:
Tw,tank = 38.9'C (water storage tank temp - mixed, half-height)
Tcomp,base = 41.1'C (compressor base temp)
Electrical:
Current = 6.1 A
Voltage = 223V (single phase, ~50Hz)
This run was performed at a mass charge calculated from the internal volume at operating conditions - internal software. The cold start-up mass was used in this trial.
I'm very interested in hearing your comments on this unit.
The particular test unit is undersized on the condenser, in my view. It is an early test machine, using a tube-in-tube condenser, TXV & scroll compressor. Typically, under a dynamic heat-ramp test, the heat-up power to raise the storage tank water temp from ambient to hot temp temp is slightly lower than the compressor performance tables would predict. I have always suspected that this stemmed from a slightly under-sized condenser. The low sub-cooling value seems to bear this out.
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Re: AWHP superheat & sub-cooling
Quote:
Evaporator:
Te,sat = 13'C (sat temp)
Te,sup = 20.8'C (vapour exit temp)
Ta,in = 24.9'C (air inlet temp)
va,in = 3.5 m/s (air inlet face velocity to evap face)
Ta,out = 21.5'C (air outlet temp)
va,out = 9.5 m/s (air outlet velocity - fan discharge)
Condenser:
Tc,sup = 64.3'C (superheated vapour inlet to condenser)
Tc,sat = 50'C (condenser sat temp)
Tc,sc = 43.7'C (liquid exit temp)
Tw,out = 39.8'C (water outlet temp)
Other:
Tw,tank = 38.9'C (water storage tank temp - mixed, half-height)
Tcomp,base = 41.1'C (compressor base temp)
Electrical:
Current = 6.1 A
Voltage = 223V (single phase, ~50Hz)
Evaporator:
dT = 24.9-21.5 = 3.4K/6.1F
TD = 24.9-13 = 11.9K/21.4F
SH = 20.8-13 = 7.8K/14F
Appr = 21.5-13 = 8.5K/15.3F
Condenser:
dT = 39.8-38.9 = 0.9K/1.6F
TD = 50-38.9 = 11.1K/20F
SC = 50-43.7 = 6.3K/11.3F
Appr = 50-39.8 = 10.2K/18.4F
I am assuming that the Tw,tank is the same as the cond entering water temp, although this may not be the case.
The subcooling at 6.3K is well within what I would consider to be a normal range (5.5-8.5K). This is largely a matter of refrigerant charge.
As to undersize/oversize, these are terms that are relative to accepted standards. Are there in fact any accepted standards for this type of system?
All seems to be working well at this point. It will be interesting to see how these numbers compare with (all else being equal) a much hotter incoming water temp, as will be seen at the end of the heating cycle.
An additional temp which should be monitored/recorded would be the discharge line temp near the compressor.
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Re: AWHP superheat & sub-cooling
Thanks so much for your comments.
Quote:
I am assuming that the Tw,tank is the same as the cond entering water temp, although this may not be the case.
This is a close-enough approximation & within the range of experimental measurement uncertainty.
Quote:
The subcooling at 6.3K is well within what I would consider to be a normal range (5.5-8.5K). This is largely a matter of refrigerant charge.
The current test point (Tc,sat=50'C) is around mid-range for the current designs. The amount of sub-cooling increases as the heating cycle advances. Towards the point Tc,sat~70-75'C, the sub-cooling effect is far more pronounced - in excess of 10-12K. This is what makes me think that the condenser is under-size.
Quote:
As to undersize/oversize, these are terms that are relative to accepted standards. Are there in fact any accepted standards for this type of system?
To be honest, the standards that seem to be out there, for this kind of machine, seem to be locally-applied, rather than an industry-wide standard. Most of it seems to be still manufacturer-specific, as far as I can determine.
Quote:
All seems to be working well at this point. It will be interesting to see how these numbers compare with (all else being equal) a much hotter incoming water temp, as will be seen at the end of the heating cycle.
Ok - good. What I'll do next is to do a set of tests at fixed points across the whole heating cycle & feed back. This will allow a more clear picture of the overall performance.
Quote:
An additional temp which should be monitored/recorded would be the discharge line temp near the compressor.
The Tc,sup = 64.3'C (superheated vapour inlet to condenser) should be re-named as 'superheated vapour exit from compressor', as this is the point where it is physically measured. I'm used to using the figure for condenser performance checks & so lumped it there. Apologies.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
Ok - good. What I'll do next is to do a set of tests at fixed points across the whole heating cycle & feed back. This will allow a more clear picture of the overall performance.
Excellent. Then we can see the changes throughout the system that occur due to the incoming water temperature rising.
There should also be a similar set of tests (start of cycle/end of cycle) with the incoming air at 35C, as this is the other design extreme.
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Re: AWHP superheat & sub-cooling
Quote:
There should also be a similar set of tests (start of cycle/end of cycle) with the incoming air at 35C, as this is the other design extreme.
May have to wait a while for that - we're in the rainy season at present. Probably get up to ~30'C in the next few days, with lots of humidity. (Fill up water buckets from the evap drain - lol)
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
May have to wait a while for that - we're in the rainy season at present. Probably get up to ~30'C in the next few days, with lots of humidity. (Fill up water buckets from the evap drain - lol)
I recall the weather extremes in that area of the world. When I was over there, people were shooting at me, so that made it even more uncomfortable... lol
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Re: AWHP superheat & sub-cooling
You really should have some means of regulating the intake air temp to simulate the two extremes that you are designing for.
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Re: AWHP superheat & sub-cooling
That's true.
I'm presently planning to relocate from SEA back to my homeland. Once settled, I plan to set up the lab with some level of environmental control - it has to be done. To add hot air is not a problem, it's more when you want to cool the air to just above 0'C, that it gets a little more costly.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
That's true.
I'm presently planning to relocate from SEA back to my homeland. Once settled, I plan to set up the lab with some level of environmental control - it has to be done. To add hot air is not a problem, it's more when you want to cool the air to just above 0'C, that it gets a little more costly.
What you need is a good heat pump. ;)
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Re: AWHP superheat & sub-cooling
^ Hahaha... got 4 of those already. Point taken... :D
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Re: AWHP superheat & sub-cooling
A few thoughts on using only AWHP's to create a hot/cold test capability.
Air cooling
Using a air-source AWHP, where the output air-stream is partially re-circulated back to the evap inlet would bring the air temp down. I wonder how far this could go, though - if ducted properly?
Air heating
Use the hot water storage to pre-heat the incoming air-stream, or use a bank of direct-element heaters to raise the incoming air stream. Ducted flow.
Any thoughts?
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
A few thoughts on using only AWHP's to create a hot/cold test capability.
Air cooling
Using a air-source AWHP, where the output air-stream is partially re-circulated back to the evap inlet would bring the air temp down. I wonder how far this could go, though - if ducted properly?
Aside from the obvious coil freezing problems:
As the temp drops the heat load drops, so eventually the TXV would start hunting and then uncontrolled flooding.
The orifice could be downsized, but the smaller the orifice the slower the pulldown.
The temp could theoretically keep dropping until the compressor reached its lower limit, where it just can't pull a deeper vacuum or the liquid temp coming back from the condenser engages the entire coil in flashing.
If you have a target temp you want to stop at you could do this with an evaporator pressure regulator (EPR) valve.
Quote:
Originally Posted by
desA
Air heating
Use the hot water storage to pre-heat the incoming air-stream, or use a bank of direct-element heaters to raise the incoming air stream. Ducted flow.
A hot water coil could be used easily enough.
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Re: AWHP superheat & sub-cooling
Actually, nothing so exotic is needed. I'm thinking a cheap window A/C could be modified to provide the heating and/or cooling.
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Re: AWHP superheat & sub-cooling
Good idea. A window-rattler.
Isn't this the beauty of being in the HVAC&R game... lol... :)
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Re: AWHP superheat & sub-cooling
The performance tests from Tc,sat = 40'C through 75'C, in steps of 5'C, are done. I need to tabulate & graph the results. Hope to have these out this weekend.
At Tc,sat=40'C, the condenser sub-cooling is 3.8K & goes up to 15.2K at Tc,sat=75'C !!! :eek:
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
The performance tests from Tc,sat = 40'C through 75'C, in steps of 5'C, are done. I need to tabulate & graph the results. Hope to have these out this weekend.
At Tc,sat=40'C, the condenser sub-cooling is 3.8K & goes up to 15.2K at Tc,sat=75'C !!! :eek:
Now you know what receivers are for.
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Re: AWHP superheat & sub-cooling
What was the leaving water temp at Tc,sat=75C?
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
What was the leaving water temp at Tc,sat=75C?
Tw,out = 60.9'C @ Tc,sat=75'C
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
At Tc,sat=40'C, the condenser sub-cooling is 3.8K & goes up to 15.2K at Tc,sat=75'C !!! :eek:
Quote:
Originally Posted by
desA
Tw,out = 60.9'C @ Tc,sat=75'C
Hmmm... 15.2K subcooling with 14.1K approach.
This would seem like an opportune moment to test the excess subcooling theory. You could remove refrigerant until the subcooling is 8.5K and see if the approach increases or decreases.
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Re: AWHP superheat & sub-cooling
^ Can you explain a little further on this, please?
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Re: AWHP superheat & sub-cooling
You were speculating earlier about using part of the condenser for excess subcooling. You now have excess subcooling backing up into the condenser.
If that excess subcooling is beneficial, then removing it would increase the approach temp at Tc,sat=75C.
If that excess subcooling is not beneficial, then removing it would decrease the approach temp at Tc,sat=75C.
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Re: AWHP superheat & sub-cooling
Ok, fair-enough.
Let me finish the full test table & put it up (wip). We can then see the progression the whole way through the heat-up cycle. I also have the dynamic ramp test from the previous day, to measure overall (average) heating rate under that mass condition.
We can then make a solid decision on how much charge to remove.
I value your input greatly - it is an extremely valuable exercise for me.
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Re: AWHP superheat & sub-cooling
Sorry, I don't mean to rush you. You will of course, need a full spectrum of test tables for a base, in order to quantify the value of improvements.
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Re: AWHP superheat & sub-cooling
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Re: AWHP superheat & sub-cooling
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Re: AWHP superheat & sub-cooling
We can be as critical of this design as we like. :)
It is a machine I bartered in trade for consulting work performed for a now defunct heat-pump builder. It is not my design, I've just fine-tuned its performance a little & tested its robustness under 3rd world conditions. I use it as my test basis on which to benchmark my next machine, which is almost complete.
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Re: AWHP superheat & sub-cooling
Note that the data speaks with a forked tongue... :)
You will observe that the Tw,out & T,tank values cross at some point. This should be non-physical & will definitely affect the condenser Approach calculation.
It is something I've observed in earlier trials at one of my clients. In my view the temp probe for the water outlet from the condenser, although insulated, is losing accuracy as temp increases, whereas the storage water temp is mixed & therefore more representative.
I will upload a plot showing both the conventional (Approach=Tc,sat-Tw,out) & modified (Approach'=Tc,sat-T,tank), for clarity. It makes a huge difference in interpretation.
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Re: AWHP superheat & sub-cooling
http://i28.tinypic.com/2hi3r6o.png
http://tinypic.com/r/2hi3r6o/3
The value (-Cross)=Tc,exit-Tw,out. This is a critical parameter for heat-exchanger design purposes.
(-Cross)'=Tc,exit-T,tank
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Re: AWHP superheat & sub-cooling
Hi Nike123, I see you lurking... welcome to comment... :)
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Re: AWHP superheat & sub-cooling
Update - water calibration curve:
What I have done is to add an additional thermocouple probe onto the condenser water outlet pipe. I will calibrate the two Tw,out probes against the T,tank probe tomorrow & adjust the experimental results accordingly.
Practically, though, I'd expect the useful data for the Approach to be for (Approach) to the left of the temp crossing point & for (Approach)' to the right of the temp crossing point. Let's see what skewing the calibration curves bring out.
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Re: AWHP superheat & sub-cooling
We have been assuming that T,tank is the same as Tw,in.
It is not possible for Tw,in (T,tank) to be higher than Tw,out.
Nor is it possible for Tw,in (T,tank) to be higher than Tc,exit.
I tend to believe the problem is T,tank.
To my thinking, it would be far preferable to measure Tw,in and assume that it is the same as T,tank than the reverse.
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Re: AWHP superheat & sub-cooling
The TXV is working very well despite the increase in liquid pressure. That's very good news.
At the end of cycle the compressor discharge temp is running dangerously high (103.9C). Since we are riding the upper limits of the compressor, we need to provide maximum cooling for it. The compressor is cooled by the superheated vapor. Therefore we need to run the lower limits of acceptable superheat. Going by Magoos rule, at end of cycle the TD is 25.2-14=11.2TD.
11.2*0.6=6.72SH.
The TXV superheat could be lowered to about 6.72K from its current 7.9K. This would lower the compressor discharge temp.
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Re: AWHP superheat & sub-cooling
^Good point. Thanks for that. I'll drop that down a tad to suit.
What maximum discharge temp would you be comfortable with off a scroll compressor?
The Copeland document AE-1263-R3 states that discharge temp measured 6" from the compressor discharge should be max ~ 250'F (121.1'C), with internal compressor exit maxima no higher than 300'F (148.89'C).
On that basis, we still have a little spare room.
The other limits along the way are as follows:
1. Motor winding limit : 325'F (162.78'C)
2. Oil breakdown limit : 325'F (162.78'C)
3. Compressor oil film limit : 310-325'F (154.4-162.78'C)
4. Sump temperature limit : 200'F (93.3'C)
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
Gary
We have been assuming that T,tank is the same as Tw,in.
True... thanks for the reminder - was getting lost in there for a second.
Quote:
It is not possible for Tw,in (T,tank) to be higher than Tw,out.
Nor is it possible for Tw,in (T,tank) to be higher than Tc,exit.
I tend to believe the problem is T,tank.
To my thinking, it would be far preferable to measure Tw,in and assume that it is the same as T,tank than the reverse.
Point taken.
The problem with these type K temp probes is that the accepted experimental uncertainty is +- 2.2'C. Add to that positioning inaccuracy, tank mixing & so forth - & we have a quagmire.
I plan to run up a calibration trial on all 3 probes tomorrow. May have to start with ice & go on up to boiling to try & find out the bias of each... headaches.
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Re: AWHP superheat & sub-cooling
Quote:
Originally Posted by
desA
^Good point. Thanks for that. I'll drop that down a tad to suit.
What maximum discharge temp would you be comfortable with off a scroll compressor?
Being from the old school, I was taught that if you spit on the discharge line and it boils you have a problem. But it seems that compressors are more tolerant these days.
Still, I like to keep the discharge a little cooler to ensure the compressor lives a long and happy life.
And a lower superheat picks up more heat in the evap, thus increasing COP.
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Re: AWHP superheat & sub-cooling
^ Points well taken.
I'll set the superheat down in the morning. That's sound advice.
The scroll compressor concept seems to be doing fairly well, it seems. Copeland seem to be continually revising their operational envelopes, based on field feedback.
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Re: AWHP superheat & sub-cooling
http://i26.tinypic.com/250ou8n.png
http://tinypic.com/r/250ou8n/3
What would be your comments on this chart?
(-Cross)=Tc,exit-Tw,out
From my heat-exchanger design experience, I would say that the condenser performance is beginning to drop off drastically after Tc,sat = 50'C, based on (-Cross), although the Approach drop-off seems to occur around Tc,sat = 55'C. The SC shows a rapid rise after Tc,sat = 55'C as well.
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Re: AWHP superheat & sub-cooling
I would attribute the dropoff in performance to the excessive subcooling. Liquid expands when heated. The liquid has no receiver to expand into, therefore it backs up into the condenser, taking up valuable space.