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Re: How to increase pool heater capacity?
This has been a fustrating job. The client has had 2 x 6kw inline electric heaters installed. :mad:
Therefore with one underspec unit with components that have dragged the compressors capacity lower and possibly heat loss underground the poor client has had to make this call. The suppliers of the unit still say that it is a 30 kw(nom) , but the client is loosing business and had to do what she did.
I might never know what the fault(s) is (are).
Thanks again for all your help and sorry to have wasted your time. I have learnt a lot which hopefully I can apply to the next job.
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Re: How to increase pool heater capacity?
If I were to state that at least one of the more well-known names in air-to-water heat-pumps has a make-up electrical element heater installed 'inside the heat-pump unit' itself - would that surprise you?
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Re: How to increase pool heater capacity?
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Re: How to increase pool heater capacity?
Its a shame we won't be able to get to the bottom of this. It could be something as simple as too much refrigerant in the system.
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Re: How to increase pool heater capacity?
Or if you go back to desA's original post was it sized correctly in the first place.Best to do a general rule of thumb before going to a whole lot of bother on this type of system.
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Re: How to increase pool heater capacity?
Yes it was sized correctley. A 24kw unit used to do the job. The client wanted to go up a size so a 30kw was installed. This "30kw" doesnt obtain temps that the 24kw obtained.
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Re: How to increase pool heater capacity?
Quote:
Originally Posted by
Drew
Yes it was sized correctley. A 24kw unit used to do the job. The client wanted to go up a size so a 30kw was installed. This "30kw" doesnt obtain temps that the 24kw obtained.
Were these two heat pumps from the same manufacturer?
If so, then something else does seem to be amiss. If same manufacturer, did their technology change in the interim?
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Re: How to increase pool heater capacity?
Quote:
Originally Posted by
Drew
Couldnt the HP be to high due to the suppliers insisting on a 3 degree TD across the coil?If I increase the water flow rate the HP would drop( incresaing the compressors capacity) , the TD would decrease , but overall the system would deliver more heat?
After this thread & the ongoing discussions, I decided to develop an equation which describes how a generic condenser operates under given parameter changes.
What seems to come from this is the following:
- decrease water flowrate;
- water outlet temp rises;
- condenser saturation temp required reduces.
- for the SAME heat-transfer rate (kW)
Basically, unless the change in water flowrate drastically alters the water-side heat-transfer coefficient in the condenser, no net additional heat-transfer is effected by tweaking the water flow-rate - it is an illusion.
The equation for water heat-balance is:
q' = m'w*Cpw*dTw
where :
q' = heat-transfered to the water [W]
m'w = water mass flowrate [kg/s]
Cpw = water specific heat [J/kg.K]
dTw = Tw,o - Tw,i
Tw,o = water outlet temp ['C]
Tw,i = water inlet temp ['C]
For a fixed heat-transfer (q'), the m'w & dTw values merely trade off against each other.
So, by having a hotter Tw,o at lower flowrate, no additional heat is actually transferred - it is an illusion, I'm afraid.
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Re: How to increase pool heater capacity?
Yup, thats the way it is. You cannot create energy out of nothing.:D
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Re: How to increase pool heater capacity?
If the heat transfer remained the same then the high side pressure would remain the same. The fact that the high side pressure increases says that lowering the water flow decreases the heat transfer.
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Re: How to increase pool heater capacity?
Quote:
Originally Posted by
Gary
If the heat transfer remained the same then the high side pressure would remain the same.
The governing equation for a condenser seems to show that under same heat-transfer condition, as stated above, that the Tsat of the condenser will actually drop slightly, when flow is reduced & Tw,out increased.
The system floats on the inlet temperature of the external cooling medium. This also presumes no fundamental heat-transfer change in that process.
I'd be very interested in why the high side pressure should remain the same, if the Tsat for equilibrium is to reduce slightly - I'd expect it, under equilibrium conditions, to reduce slightly in line with the reducing Tsat. This is in the order of a few degrees celsius, but it is predicted.
:)
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Re: How to increase pool heater capacity?
When an actual (accurate) measurement shows one result and a calculation shows another result, there is a flaw in the calculation.
A measurement beats a calculation every time.
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Re: How to increase pool heater capacity?
Drew might have achieved greater "q" due to decrease of "m'v" being less than increase of "dTw".
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Re: How to increase pool heater capacity?
Dew might have achived greater "q" due to decrease of "m'w" being less than increase of "dTw".
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Re: How to increase pool heater capacity?
Quote:
Originally Posted by
Yuri B.
Drew might have achieved greater "q" due to decrease of "m'v" being less than increase of "dTw".
Since q in fact decreased, as evidenced by the increase in high side pressure, I'm thinking that you have this backwards, but you are on the right track.
What we are missing here is the measured values of mw. A decrease in q tells us that the value of mw decreased faster than the value of dTw increased.
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Re: How to increase pool heater capacity?
Quote:
Originally Posted by
desA
The governing equation for a condenser seems to show that under same heat-transfer condition, as stated above, that the Tsat of the condenser will actually drop slightly, when flow is reduced & Tw,out increased.
The system floats on the inlet temperature of the external cooling medium. This also presumes no fundamental heat-transfer change in that process.
I'd be very interested in why the high side pressure should remain the same, if the Tsat for equilibrium is to reduce slightly - I'd expect it, under equilibrium conditions, to reduce slightly in line with the reducing Tsat. This is in the order of a few degrees celsius, but it is predicted.
:)
The flaw in all this is the assumption of fixed heat transfer. The heat transfer is not fixed.
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Re: How to increase pool heater capacity?
At this point I would strongly suspect that the system is overcharged, with excess liquid refrigerant backing up into the condenser, limiting it's ability to transfer heat. A simple subcooling measurement would confirm or deny this.
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Re: How to increase pool heater capacity?
Quote:
Originally Posted by
Gary
The flaw in all this is the assumption of fixed heat transfer. The heat transfer is not fixed.
Thanks for that. I'm very interested in understanding why the heat-transfer should be variable, & the high-pressure remain constant.
The equation, of course, also allows for a changing heat-transfer. I'm interested in the physics/thermodynamics & experimental observations for the reasoning behind this.
I bow to your many years of experience in this - no contest - & would be interested in why this occurs.
I've resolved the governing equation in terms of heat-transfer as a function of the remaining variables. The water mass flowrate appears in two places:
1. Linear increasing term;
2. (1-exp(-a)) term;
This shows a competition between two scenarios for heat-transfer as a function of water mass-flow. I'll plot it out.
This has got me intrigued. I'll scurry off to my laboratory heat-pump & test out the real system response over a number of scenarios. It's actually an age-old chestnut & I'd like to get completely to the bottom of it.
:eek:
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Re: How to increase pool heater capacity?
Quote:
Originally Posted by
desA
Thanks for that. I'm very interested in understanding why the heat-transfer should be variable, & the high-pressure remain constant.
Possibly I was not clear in this. You were saying that the heat transfer must remain constant... and I was responding that if the heat transfer were constant, then the high side pressure would be constant. In fact, neither is constant.
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Re: How to increase pool heater capacity?
Quote:
Originally Posted by
Gary
Possibly I was not clear in this. You were saying that the heat transfer must remain constant... and I was responding that if the heat transfer were constant, then the high side pressure would be constant. In fact, neither is constant.
Fair enough. :D
Can I press a little further in, if I may be so bold...
(1) Why is neither constant?
(2) How are heat-transfer & high-side pressure related?
In a system, some component interaction around the refrigeration loop can sometimes showcase some very interesting effects.
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Re: How to increase pool heater capacity?
The pressure of refrigerant is directly related to its saturation temperature. As the saturation temp rises, the pressure rises. As the saturation temp falls, the pressure falls.
The purpose of the condenser is to transfer heat from the refrigerant to the water. If heat is transferred faster (increased q), the refrigerant is cooled. In other words its saturation temperature falls, as does its pressure.
If the rate of heat transfer is less (reduced q) the refrigerant retains more heat and its saturation temp rises, as does its pressure.
In this case the suppliers decreased the water flow. As a result, the pressure increased, therefore the saturation temp increased, which tells us there is less heat transfer (lower q).
If you factor lowered q into your formula, you will see that water flow and delta-T cannot be an equal trade-off. In order for q to be lower, the decrease in water flow must exceed the increase in dT.
Decreasing the water flow was counter-productive.
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Re: How to increase pool heater capacity?
^ Thanks very much for your explanation, Gary. :)
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Re: How to increase pool heater capacity?
Yes , the fact that the high side pressure increases says that lowering the water flow decreases the efficiency of the heat exchanger forcing the compressor to pump to a higher pressure to condense . With the lowered refrigerant flow rate the compressor and system balance at a new point with adjusted THR and system performance.
And visa versa, increasing flow rate would create a more efficient heat exchanger and would lower the head pressure increasing capacity.
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Re: How to increase pool heater capacity?
Quote:
Originally Posted by
desA
This has got me intrigued. I'll scurry off to my laboratory heat-pump & test out the real system response over a number of scenarios.
:eek:
ha ha thats funny. I can just picture it!:D
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Re: How to increase pool heater capacity?
Heat absorbed = Heat transfered + heat of compression = heat rejected
If we alter the 'heat transfered + heat of compression' part of the formula (compressor capacity), by altering the operating pressures the heat 'in' would still equal the heat 'out'.
The compressor will deliver a certain capacity based on the conditions that are created by the 2 heat exchangers. As the oncoil temp increases, the head pressure rises. As the head pressure rises the compressors capacity decreases.As the compressors capacity decreases there is less energy to reject . The compressor and condenser balance at a new pressure which equates to the capacity.The three components individually now have this capacity which becomes the changed system capacity. The system is dynamic.
Again
Heat absorbed (kw , evap)= heat transered + heat of compression (kw ,compressor) = heat rejected (kw, cond)
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Re: How to increase pool heater capacity?
Quote:
Originally Posted by
Gary
In this case the suppliers decreased the water flow. As a result, the pressure increased, therefore the saturation temp increased, which tells us there is less heat transfer (lower q).
Decreasing the water flow was counter-productive.
Decreasing the flow rate was counter productive because it forced the compressor to a higher SCT and at this higher point the compressor has less capacity so less heat has to be rejected by the condenser. This is why there is less heat transfer?
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Re: How to increase pool heater capacity?
Quote:
What seems to come from this is the following:
- decrease water flowrate;
- water outlet temp rises;
- condenser saturation temp required reduces.
- for the SAME heat-transfer rate (kW)
I do apologise - been re-reading my typing here (had a niggling thought today, thought I'd check) & checking my simulation - a typing error had crept in. It should have read as follows:
- decrease water flowrate;
- water outlet temp rises;
- condenser saturation temp required increases.
- for the SAME heat-transfer rate (kW)
This would now seem to agree with what followed my original comment. Sorry about the confusion.
Following on the Tc,sat change should be an appropriate Pc,sat change.
For small flow-rate changes, the change in equilibrium Tc,sat is not too much, but at very low flows, the Tc,sat can rise substantially. I've actually seen this effect in my lab heat-pump, when the water flow is backed off hard, or stopped. The pump then trips on the high-pressure safety switch fairly promptly.
Practically, as Gary stated, the actual heat-transfer in the condenser most probably doesn't remain completely constant, but the analysis offers useful information nevertheless.
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Re: How to increase pool heater capacity?
Quote:
Originally Posted by
desA
- for the SAME heat-transfer rate (kW)
I might be loosing the plot here, but just to make sure I understand: If the head pressure climbs the compressor delivers less energy, the heat exchanger will adjust to the new capacity and therefore exchange less kw?
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Re: How to increase pool heater capacity?
Quote:
Originally Posted by
Drew
I might be loosing the plot here, but just to make sure I understand: If the head pressure climbs the compressor delivers less energy, the heat exchanger will adjust to the new capacity and therefore exchange less kw?
Will the compressor deliver less energy, or reset itself to a higher energy level, with consequent higher delivery to the condenser?
If this is correct, then surely the condenser output would rise & the heat-pump folks would have been correct all along?
:confused:
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Re: How to increase pool heater capacity?
If you look at a compressor capacity chart you will see that the higher the saturated condensing temp the less the capacity . The lower the suction the less the capacity. To obtain the best capacity from a compressor we keep the suction as high as possible and the head as low. These pressures are governed by ambients and design product temp. Within reason we try to stick within these parameters. Its also a matter of cost and rules of thumb.
The reason that the capacity is reduced with high head pressure and low suction is due to the reduced refrigerant flow rate with these pressures. With a reciprocating compressor the piston must never hit the valve plate so we have a gap (clearance volume) as a saftey. On the piston downstroke the refrigerant in the clearance, which is actually the same as the head pressure,expands before new refrigerant can be drawn in. If the pressure is high a lot of piston downstroke is wasted before drawing in new refrigerant. If the suction is low it is even worse. Not only is a lot of the downstroke wasted overcoming the high head pressure , but the suction valve will open even later. These issues reduce refrigerant flow through the system. The lower the flow rate the lower the capacity.
Ive read that on average 1kg/hr of R22 will absorb about 50w of heat at the evaporator. If we reduce this flow rate the capacity will decrease.
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Re: How to increase pool heater capacity?
Quote:
Originally Posted by
Drew
If you look at a compressor capacity chart you will see that the higher the saturated condensing temp the less the capacity . The lower the suction the less the capacity. To obtain the best capacity from a compressor we keep the suction as high as possible and the head as low. These pressures are governed by ambients and design product temp. Within reason we try to stick within these parameters. Its also a matter of cost and rules of thumb.
For condenser heat output, at increased hp temp/pressure, you're correct in that the total heat output should reduce slightly. This is true. So, it should reduce then... fair-enough, on that logic.
Sidebar:
The compressor input is predicted to rise, though - not fall with rising Tsat,cond (at same Tsat,evap).
For a heat-pump, as the Tsat,cond rises, the percentage of the total condenser output, moves from being evaporator-dominated, to compressor-dominated. Take a Copeland chart & try this. Simulators predict the same thing.
Downside:
The problem with only looking at one aspect of the system like this is that we've forgotten about the other parts. Once the condenser stabilises, does the evaporator re-adjust itself to a new equilibrium?
Practical note:
One of the problems with adjusting water flow in such a way as to raise Tsat,cond, is that it becomes a problem to reach water temps of more than 60'C, since this can push compressors above their recommended safe operating range (65'C shown in Copeland US data-sheets, 75'C in European data sheets). For R-134a it becomes a real balancing act up at that temp range, I can tell you. I try, if possible, to keep as low an approach within the condenser, as possible, to compensate for this problem.
In a pool-heater this is generally not an issue, as you still have ample temp margin available - but, for an AWHP, this can become an issue.
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Re: How to increase pool heater capacity?
Quote:
Originally Posted by
Drew
Heat absorbed = Heat transfered + heat of compression = heat rejected
Using your formula:
If the heat rejected is less, then the heat absorbed must be less and the heat transferred must be less.
Which of these factors is the controlling factor? None of them and all of them. A change in any part of the system is a change in every part of the system.
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Re: How to increase pool heater capacity?
Well, this has indeed been a fascinating thread. I've certainly learned a great deal.
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Re: How to increase pool heater capacity?
In order to properly evaluate a system, we need to view it as a heat transfer chain, with the goal being to identify and strengthen the weakest link.
For example: If we reduce the airflow through an evaporator, the slower moving air has more time to cool as it passes through the coil, thus the delta-T increases. At the same time, the SST has decreased, verifying that less heat is being transferred due to the reduced air volume.
Assuming a fixed speed compressor, this tells us that an increase in delta-T must indicate reduced airflow and that reduced airflow = reduced heat transfer. In other words, high delta-T is an indicator of airflow problems.
There are similar indicators for each link in our heat transfer chain.
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Re: How to increase pool heater capacity?
Quote:
Originally Posted by
desA
Sidebar:
The compressor input is predicted to rise, though - not fall with rising Tsat,cond (at same Tsat,evap).
the rise in input doesnt out do the loss in refrigeration effect and the result is a loss.
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Re: How to increase pool heater capacity?
Quote:
Originally Posted by
Gary
In order to properly evaluate a system, we need to view it as a heat transfer chain, with the goal being to identify and strengthen the weakest link.
Well put.
It just takes one undersized component to drag the system down to a less effeicient capacity.
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Re: How to increase pool heater capacity?
Quote:
Originally Posted by
desA
Sidebar:
The compressor input is predicted to rise, though - not fall with rising Tsat,cond (at same Tsat,evap).
In fact, rising Tsat,cond is generally accompanied by rising Tsat,evap... yet we know there is a drop in capacity. The loss must therefore be in refrigerant mass flow.
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Re: How to increase pool heater capacity?
Ran up a test on the air-to-water heat-pump in my lab today.
I let the system stabilise at a full-flow condition on the water side, for around 30 minutes & recorded system readings. Then reduced the water flowrate substantially, let system stabilise to same tank water temp (allowed 45 minutes) & took new readings.
The trends observed were at follows:
1. Tsat,con rose from 48'C to 53.8'C on gauge (+5.8'C);
2. Psat,con rose from 1150 kPa to 1350 kPa on gauge (+200kPa);
3. Tsat,evap rose 0.5'C (within experimental error)(on gauge);
4. Compressor amperage rose 12.2% (actual recorded, marginally different from Copeland data sheet);
5. dT,water rose from 1'C to 5.7'C (poor sensor placement - indicator only)
6. First test : Tw,in=38.0'C ; Tw,out=39.0'C ; Tc,sup=60.2'C; Tc,exit=43.6'C ; Tw,tank=37.1'C (sensor position & mixing); Tevap,sup=23.4'C;
7. Second test : Tw,in=38.8'C ; Tw,out=44.5'C ; Tc,sup=65.8'C ; Tc,exit=48.7'C ; Tw,tank=38.2'C (sensor position & mixing); Tevap,sup=22.6'C;
When calculations & curve-fits are performed against the Copeland Europe predictions in 'Select 7.1', the following information emerges:
a. Condenser output load reduces (-3.06%);
b. Compressor input power increases (+14.522%);
c. COP,hp reduces substantially (-15.355%)
Air on/off evaporator coil remained similar in both tests (performed in same test run). The slight difference in recorded compressor power increase & Copeland predicted values is attributed to a system refrigerant charge still slightly under the optimum conditions. Optimum charge determination is still in progress for this heat-pump.
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Re: How to increase pool heater capacity?
The approach temp seems to have remained relatively stable (Tsat and water out temp rising in unison), as I would expect, but not knowing the actual Tsat and leaving water temp I am wondering what your approach temp is.
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Re: How to increase pool heater capacity?
Quote:
The trends observed were at follows:
1. Tsat,con rose from 48'C to 53.8'C on gauge (+5.8'C);
2. Psat,con rose from 1150 kPa to 1350 kPa on gauge (+200kPa);
3. Tsat,evap rose 0.5'C (within experimental error)(on gauge);
4. Compressor amperage rose 12.2% (actual recorded, marginally different from Copeland data sheet);
5. dT,water rose from 1'C to 5.7'C (poor sensor placement - indicator only)
6. First test : Tw,in=38.0'C ; Tw,out=39.0'C ; Tc,sup=60.2'C; Tc,exit=43.6'C ; Tw,tank=37.1'C (sensor position & mixing); Tevap,sup=23.4'C;
7. Second test : Tw,in=38.8'C ; Tw,out=44.5'C ; Tc,sup=65.8'C ; Tc,exit=48.7'C ; Tw,tank=38.2'C (sensor position & mixing); Tevap,sup=22.6'C;
There we are, Gary - I've added in the missing data you need.
I'll add in a few more items, if you'd like to crack it open - I'd love to hear your thoughts.