How to increase pool heater capacity?

[Drew]

A company has installed a heat pump style pool heater for a client who has complained that it just doesnt reach temerature ( 30.5 degrees C).
I am trying to increase the capacity as much as i can without changing any components. The suppliers of the heat pump have said that i should decrease the water flow rate through the heat exchanger? This they say will increase my head pressure and increase the temperature difference across the heat exchanger. 'This would increase my capacity.' While this initially sounded right i now, after thinking about it, disagree. If we lift our HP the compressors capacity decreases. At the moment we have a 47 degreeC HP with a water in of 27 and out of 30.

Shouldnt we rather increase the water flow rate through the heat exchanger , lowering our HP , which is an increase in capacity?. Even though our HP is lower and our TD will be less due to the increased flow rate and the improved compressor capacity will be an overall system improvement?

In other words if I lower my HP from 47 degreesC to 40 DegreesC I increase my compressor capacity from 20kw to 25kw. This i can acheive by increasing the water flow rate which increases the capacity of my water cooled condenser.

Therefore having a Standard 40 DegreeC head pressure with its corrosponding flow rate is better from a heating capacity point of view than a high head pressue with its slower flow rate???

[desA]

What compressor (model) are you using?
What is the evaporator saturation pressure/temp?
What is your condenser saturation pressure/temp?

Let's first see what the thermodynamics tells us, before playing too much.

By-the-way, increasing water flow velocity within the allowed pressure-drop generally increases the heat-transfer in the heat-exchanger (condenser), due to an increase in the water-side heat-transfer coefficient, as well as an increase in the log-mean-temp-difference (dTlm).

Slowing down flow purely trades off with a larger dT, water, but reduces dTlm - not where you want to go, generally.

Practically though, the following questions will tell us if the pump is large-enough for the job at hand:
1. Pool volume, or surface area x depth;
2. Days to heat;
3. Covered, or bare?
4. Ambient air temp;
5. Water starting temp.

The pump may just be way too small & then there is precious little you can do without adding in a larger heat-pump.

[nike123]

At the moment we have a 47 degreeC HP with a water in of 27 and out of 30.

I would say that you have dirty condenser, noncondensables in condenser or overcharge since TD of condenser is to high! With that water in temperature your condensation temperature should be 42°C. Check condenser for dirt.
Check system for noncondensables or overcharge.
What is subcooling?

[Drew]

Thanks for the prompt reply.

I have a feeling that we are border line with regard to capacity. However I would like to confirm that their is nothing further i can do to improve performance. The suppliers theory of decreasing the water flow rate does worry me.

The way i understand it from a technicians point of view is that the lower the HP the more the compressors capacity due to the increased refrigerant flow rate. The compressor dictates the system capacity? The lower the HP the more refrigerant flow the higher the capacity.

If I increase the flow rate through the heat exchanger I lower the HP and even though the refrigerant isnt as hot more energy is exchanged?

Therefore even though the TD across the coil is decreased the water change rate of the pool is increased and with the compressors improved capacity the overall capacity is improved?

[Drew]

I would say that you have noncondensables in condenser or overcharge since TD of condenser is to high! With that water in temperature your condensation temperature should be 42°C.
Check system for noncondensables or overcharge.
What is subcooling?

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? Currentley the HP is 47 degreesC , LP is -2 degreesC, 3 degreesC across heat exchanger. Ambient is 10 DegreesC. This is a standard 'off the shelf' pool heat pump coupled to a standard pool pump. I believe the heater is under sized , but would like to understand the theory of how the different Head Pressures and flow rates would influence capacities? Im not sure of the sub cooling.

[nike123]

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? Currentley the HP is 47 degreesC , LP is -2 degreesC, 3 degreesC across heat exchanger. Ambient is 10 DegreesC. This is a standard 'off the shelf' pool heat pump coupled to a standard pool pump. I believe the heater is under sized , but would like to understand the theory of how the different Head Pressures and flow rates would influence capacities? Im not sure of the sub cooling.

3°C across heat exchanger is not TD. That is condenser secondary side delta t (Δt).


TD is difference between SCT (saturation condensation temperature) and water in temperature and for water condenser should be around 15K.

Condenser (Δt) should be around 5K for best efficiency and it is adjusted by water flow.

[desA]

3°C across heat exchanger is not TD. That is condenser secondary side delta t (Δt).


TD is difference between SCT (saturation condensation temperature) and water in temperature and for water condenser should be around 15K.

Condenser (Δt) should be around 5K for best efficiency and it is adjusted by water flow.

A decent condenser will have a TD around 10-12'C, if at all possible. If it needs to sit above 15K, then please check the items nike123 suggested.

Perhaps you could refer to the water-side temp rise as dT,w. This is typically around 3-5'C.

To the OP, please don't forget to check that the supplied rating of the heat-pump is actually what the pool requires. Heat-pump sizing needs to be correct.

[Gary]

3°C across heat exchanger is not TD. That is condenser secondary side delta t (Δt).


TD is difference between SCT (saturation condensation temperature) and water in temperature and for water condenser should be around 15K.

Condenser (Δt) should be around 5K for best efficiency and it is adjusted by water flow.

While I agree with the above, there is one more factor to look at: Approach temperature.

The approach temp is the difference between SCT and leaving water, and is an indicator of heat transfer from the refrigerant to the water. Approach temp should be no more than 20F/11K. In this case, the approach is 47 - 30 = 17K/30.6F. This is much too high.

In general, a high approach temp means the interior surfaces of the condenser need to be acid cleaned. But it can also mean that the condenser has been installed incorrectly.

Maximum heat transfer is achieved by running the two fluids (refrigerant and water) through the condenser in opposite directions (counterflow) or across each other (crossflow).

It is possible that the water in and water out connections have been swapped, resulting in the two fluids running in the same direction (parallel flow). Parallel flow is extremely inefficient.

[Gary]

Here is one more thing to think about:

If the water connections are top and bottom, it is good practice to fill the water side of the condenser from the bottom up, pumping water into the bottom and exiting the top. This ensures that water fills every little nook and cranny eliminating any air pockets.

I'm thinking pics of the system might be very helpful here.

[nike123]

I think he should first measure water flow and calculate achieved duty, and then compare with rated data. That will tell a lot.

[laf100]

You guys appear to way ahead of me on this one, but surely if we continuously add a certain amount of heat (however small) to a body of water (however large), as long as the heat losses are less that the heat added, it must continue to rise in temp.
However slowly, would eventualy reach setpoint, would it not? Perhaps I am oversimplifying things a bit, but retetion of the added heat must be a factor in this problem?

LAF


OK- only after writing this drivel do I notice the OP is from Aus'!!! Maybe my heat loss theory is "blown out of the water" after all!!

[Drew]

Thanks guys. I will be using the above info. The flow direction of the water is an interesting one which I havent checked. The unit is new and therefore cannot have scale or a dirty condenser. Thanks for correcting me on the terminology. As a technician and not an engineer i try and keep it simple.

The suppliers of the unit say we should obtain a 3 DegreeC water temperture difference across the coil. When I obtain this the HP is at 47 Degrees. Whether this is a poor design issue or a fault is yet to be proven.The suppliers unfortunately doent seem to know much about their product either.

The way I understand it we can measure the capacity of the unit by the following :

KW= l/s x 4.19 x water temp change

My compressor delivers say 20kw @ (47SCT; -2SST)

If i lower my HP and maintain my suction it then ,on paper delivers 25kw (40 SCT ; -2SST )


If I increase my water flow rate to achieve this lower hp will their be an increase in capacity on the system in theory?

We could therefore go from:??

20kw(47SCT; -2SST) = Lower l/s x 4.19 x (30-27)

25kw (40SCT; -2SST) = Higher l/s x 4.19 x (30-28)


In other words would 25kw @ 40degreesC be more effective than 20kw @ 47degreesC?? with their adjusted flow rates.

At the end of the day we can play with flow rates and temerature difference, but they all balance out unless we increase our KW input.We cannot make energy out of nothing?

If this was an electric heat system and i added 25kw @ 47degreeesC there would be an improvement. Due to the fact that it is a refrigeration system the higher the HP the lower the capacity offered from a heating point of view.

The way i see it is that I have made the condenser more efffeicient by increasing the flow rate, which has lowered the hp which has increased the capacity of the compressor and the system overall?

If this was an air cooled condenser and I increased the air flow over the coil my HP would drop and as long as my TEXV is sized correctley for the pressure drop , I would increase the capacity of the compressor and the system overall.

I understand this might be an increase of only 10% or so , but im tying to make a point with the suppliers that decreasing the flow rate, which increases the HP and has a larger temp difference across the coil is not more effecient than a lower HP , increased flow rate and decreased temp difference.
Understanding this is a pool application which recirculates the water over a few days so the water off temp is not really that critical.

[Drew]

OK- only after writing this drivel do I notice the OP is from Aus'!!! Maybe my heat loss theory is "blown out of the water" after all!!

I live in Tasmania which has an ambient average of about 8 DegreesC at the moment.
I do believe the sytem is slightly undersized and is managing to achieve 30degreeC, but no higher. At 47(SCT) the compressor and the pool have reached the
Balance point with heat in equalling heat out. The client only needs 31 Degrees pool temp and that is why I jusy need a little more power and i thought i could achieve this by lowering the HP by increasing water flow rate, increasing the condenser capacity, gaining more Compressor capacity and overall sytem performance.
This as Ive said often, in contrary to the suppliers technical support advice of limiting the flow rate further to increase the HP?

[Magoo]

Tassi is on a similar latitude as NZ. So if ambient is at 8 'C , it is time to shut down and winterize system. Even with a thermal pool cover the heat loss from pool , add windage factor, will be beyond system to maintain 31' C the Heat pump will not cope.
The heat pump salesman will say anything to sell product and cloud the real issue.
magoo

[desA]

http://www.aquaheat.co.za/pages/calculate

Please use the above pool heat calculator to determine the 'correct' size of heat-pump required.

If your current heat-pump is new & won't reach duty, please have your client purchase additional capacity, or trade it in for the correct size unit.

[Gary]

Another think we need to check is the subcooling. Subcooling is the difference between SCT and liquid line temp at the receiver outlet (or condenser outlet if there is no receiver). The subcooling should be no more than 15F/8.5K.

High subcooling means excess liquid is backing up into the condenser, reducing the area that is available for active heat transfer. High subcooling is usually caused by overcharge.

This system definitely has a heat transfer problem. We just need to figure out why.

[Drew]

I will check the sub cooling. If the system is designed with a slightly small heat exchanger the head pressure will balance at a higher pressure limiting the capacity of the system. If i increase the flow rate, which i can, by adjusting the bypass valve, i will increase the capacity of the system. ?

[desA]

To put things a little bluntly - if you don't first determine the correct heat-load requirements & confirm that the heat-pump is the correct size, then you are really just wasting your time. Honestly.

Whether the dT,w is low/high, with water flow high/low makes little real difference - it is fine-tuning, where you're playing around within the condenser itself.

The thing you have to focus on is what is happening to the actual water in the pool - not the heat-pump. If the water is not getting hot (in new condition), it is in all likelihood (95%), undersized. If that is the case, then you are wasting your time searching within the heat-pump itself for your answers.

If the thing is not working in new condition, what on earth will it do when it is fouled?

[Gary]

Given the current heat transfer, coupled with the low delta-T, it appears that you already have a heavy flow rate and have reached a point of diminishing returns. Little can be gained by increasing the flow rate.

Once the heat transfer is increased, however, this will raise the leaving water temp, which will raise the delta-T. At that point, increasing the flow will gain more capacity.

[Drew]

I think you are right. What im trying to do is fine tune a system that doesnt really have the power no matter what i do. It is just the theory that interests me and the annoying fact that the suppliers reacon that it is a nominal 30kw , but no where on the compressors capacity curves can i find that sort of capacity. The most i can find is 25kw @ 0 SST/40SCT. ( This includes the power consumption)
I'll never get my suction above 0 with ambients like what we get in Tassie. I could probably get a little extra if i lower my HP , but i dont think it will be enough. I will check the water side for contra flow and ill check my subcooling. Thanks for the link to the heat load calc for pools.
The pool is an indoor pool for a swimming class and so it is used all year round.

[nike123]

I think, before anything, you should establish that this heat pump delivers its rated capacity and works as it should be working in given conditions. If this heat pump doesn't deliver its capacity than it must be found what is wrong and then act accordingly. What is the point in flow adjustment and pool capacity calculation if heat pump delivers only 75% of its rated capacity.
So measuring flow and temperature difference should be first thing to do. Otherwise, it is walk in the dark.
Fact that something is new doesn't mean that works as it should. I have seen many equipment coming faulty right from factory (Italian especially) with all quality check tags signed.

I still say that 47°C SCT with 27°C water in is not good and your unit has some fault.

[Drew]

Im not sure of the flow rate, but what i know is that our HP is at 47DegreesC , Suction at -2DegreesC which offers 20 kw according to the compressor manufacturers curves. The temperature difference across the coil is 3 degrees.

Therefore is the following correct:

kw=l/s x 4.19 x Temp difference


Therefore 20 = l/s x 4.19 x 3

l/s = 1,6.

So i can use the compressor capacity charts to obtain capacity of the system or use the above formula.
I was hoping that i could solve this issue by lowering the hp and gaining 5kw which could have saved the day. I will make sure the mechanical side is working correctley . The correct heat load for the pool is 30 kw and the company that sold her this unit stated that it is 30 kw. Unfortunately i can only find 20 and was hoping to squeeze out another 5 by fine tuning.

[desA]

One of the main problems with heat-pump marketing, is at what temperature the performance & COP figures are quoted.

Heat-pumps start off with tremendous performance & COP values in the startup (cold) condition, but these both decrease as the pool-water temperature rises.

It could be that the 30kW cited is a start-up condition & not the terminal (hot) condition, or even the range midpoint.

What compressor is used?

[Drew]

With regard to the installation the unit is raised about a meter off the ground . I need to check whether the water fills from the top or the bottom. Ideally we should be filling from the bottom with contra flow refrigerant. If it filled from the top or some how has an air pocket in the heat exchanger i could get the high hp with poor temp difference.

Once again if I do the KW = l/s x 4.19 x temp diff

This will tell me what the unit is delivering. We can compare this to the compressor capacity curves and then confirm whether it is working to the specs. if not find the fault. if it is then the suppliers need to pass over a true 30 kw as spec,

[nike123]

Once again if I do the KW = l/s x 4.19 x temp diff

This will tell me what the unit is delivering. We can compare this to the compressor capacity curves and then confirm whether it is working to the specs. if not find the fault. if it is then the suppliers need to pass over a true 30 kw as spec,

Yep, that is way I preach from post# 10.

[desA]

^ & ^^ Big problem then...

What if your water temperature measurements in/out followed a typical accuracy of around 1'C?

Say your cold in read 1'C high & the hot out read 1'C low? You'll end up fighting with the heat-pump supplier until the cows come home. It is very difficult to measure water temps accurately unless you have a well-calibrated test system.

If you're going to measure anything at all, do it on an average pool water value. Plot this against time, to see what the average heat input is over the whole period. You can then trace backwards to see what the heat pump is delivering - on average.

[Drew]

Thanks everyone for your help on this. I will let you know indue course of what i find. One thing is for sure ive learnt a lot !

Thanks again. Refrigeration is truely an international language and a challenge.

[desA]

^ All the very best. I'm sure you'll get it all sorted out.

[Drew]

Hey desA

I sent you a private message. i hope you get it because im not sure if i sent it right.

[desA]

^ Thanks very much, Drew. Received & answered.

Alles van die beste, kerel.

[Gary]

Therefore is the following correct:

kw=l/s x 4.19 x Temp difference


Therefore 20 = l/s x 4.19 x 3
l/s = 1,6.

Nike stated earlier that the SCT should be 42C with a delta-T of 5K, so let's give those numbers a try:

1.6 x 4.19 x 5 = 33.52 KW (as opposed to the 20 KW we are currently getting)

Are we wasting our time fine tuning the system?... or are we looking for a serious capacity-killing heat transfer problem, which the approach temperature tells us exists?

[Drew]

Thats what i will try and find out today Gary. Thyanks for the input.I'll chech the sub cooling , superheat, reclaim and recharge, chech flow direction, all after confirming what capacity the unit is delivering

[Drew]

Just to keep those interested in whats happening up to scratch:

The suppliers of the unit went to site today with their own technician (without me) and lowered the water flow rate through the heat exchanger, lifting the temperature difference across the coil to 6 C(36C off and 30C on) which lifted the SCT to 52 DegreeC.

Coincedentley after a warmish day today the unit then reached its 30.5C
Their was lots of back slapping and congratulations all around with a happy client.

However tonight temperatures drop to 2 C and we will see what the pool temp is tommorrow.

Raising the SCT by 1C lowers compressor performance by 1% (so Ive read) so by doing what they have has already lowered system performance by 2%.

I will have to wait a few days to prove this as the water temp in the pool takes a while to change.

[desA]

Be intrigued to see what has happened to the unit COP.

[desA]

The suppliers of the unit went to site today with their own technician (without me) and lowered the water flow rate through the heat exchanger, lifting the temperature difference across the coil to 6 C(36C off and 30C on) which lifted the SCT to 52 DegreeC.

So, let's see where we are:

Mean water temp = (30+36)/2 = 33'C
SCT = 52'C
Mean approach temp = 52 - 33 = 19'C !!!

I'm starting to lean towards Gary & Nike's views. Something does not add up nicely wrt the condenser.

Is the system overcharged?

[nike123]

Just to keep those interested in whats happening up to scratch:

The suppliers of the unit went to site today with their own technician (without me) and lowered the water flow rate through the heat exchanger, lifting the temperature difference across the coil to 6 C(36C off and 30C on) which lifted the SCT to 52 DegreeC.

What type of heat exchanger is in that unit?
Plate or shell and tube or else?
What is unit make and model?

Since you where there, did you checked that flow and return pipe of pool water are correctly connected at unit at their respective connections. Did you checked that maybe connections on unit are tagged wrongly in regard of counter flow of two media at heat exchanger?

[Gary]

We are all going to be very disappointed if you didn't check the water flow direction and the subcooling.

[Drew]

I will get to the bottom of this. (and let you all know)

The unit is labelled correctley with regard to water flow and the water pipe connections are correct.
What I noticed on the evaporator is that their seems to be a few heat pump model numbers on it which indicates 'one size fits all' models? I hope it isnt the same with the titanium heat exchanger. There is no point in having a compressor that delivers 25kw on paper that is dragged down with a small evap and condenser. Still need to get to site to check subcooling.

[Drew]

Hi Guys,

I still havnt been to site , but as expected , with the low ambients, the pool lost temp overnight.

My boss, did however, notice what i think could save the day. I realise we wont get the 30 kw as spec, but could increase it by quite a bit. Also the undersized, inefficient (or what ever) heat exchager isnt great. The manufacturers arent interested.

This unit was installed in place of an older one. The water pipes run into a wall and then undergound!!!
The water leaves the exchanger at say 30C , but only arrives into the pool at 28.5C. We have installed a carel thermometer in the water in and out side of the heat exchanger so the reading leaving the heat exchanger should be accurate and I hope to confirm this myself. This is also why the actual reading into the pool hasnt been worried about. We've been to focused on off and on coil temp.

Would this be true:

l/s = maybe worst @ 1 ( due to suppliers throttling water to obtain 6C across coil)

Temp difference down water pipe = 1.5C

Kw=l/s x 4.19 x temp difference down pipe.
= 1 x 4.19 x 1.5
= 6.2kw
Now, if I can assume, the heat loss to be equal on the return pipe.

Therefore 12kw???
Can this be right?

That would be a big chunk of capacity out of the unit that is already handicapped?

[Gary]

I'm thinking we need to base our conclusions upon measurements and not assumptions.

Another thing to think about: If the water is losing heat underground, then the slower it moves through the pipe the more heat will be lost. Another reason to increase the flow.

We haven't even looked at the low side of the system yet. The low side has similar indicators: delta-T, SST (saturated suction temp), TD, evap approach, superheat, etc.

The more heat in (low side), the more heat out (high side).

[Drew]

This has been a fustrating job. The client has had 2 x 6kw inline electric heaters installed.

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.

[desA]

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?

[Drew]

Not any more!

[Gary]

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.

[yinmorrison]

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.

[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.

[desA]

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?

[desA]

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.

[Drew]

Yup, thats the way it is. You cannot create energy out of nothing.

[Gary]

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.