Pool AWHP's

[aircon50]

Hi all

Fairly simple question regarding SH and SC, which I hope will result in a reasonably simple answer!
Is there a 'rule of thumb' band of temperatures to indicate that SC and SH are somewhere near correct? I know these will vary with different refrigerants, but expect them to be not too dissimilar.
For example, I have taken pressure and temp readings from a R407c pool heat pump.
Water in 22.4C
Water out 23.9C
Liquid line 31.0C
Liquid press 18.5Bar Thus SC 14.0
Evap air on 21.5C
Evap air off 14.2C
Suction line 12.3C
Suction press 3.8Bar and SH10.3

The problem is that the evaporator occasionally ices, and although the frost sensor should cut-out the compressor, it doesn't. The machine is exposed to full sun on one of three evaporator sides. The ambients, here in Tenerife are generally high enough that icing shouldn't occur, but we have another machine doing the same!
I'm not sure about SC, but SH at 10.3 seems a little high to me.

Any information would be useful
Thanks
Graham

[Bigfreeze]

You DT across your condenser is very low. By keeping such a low DT you are keeping the discharge temp low and that will keep your evap temp down. Ideally your condensing temp should be about 1-2C above your water out temp. Slow down the pump on the pool side until you have 5-7C DT and see how it runs

[nevgee]

The evap pressure 3.8bar seems low.
I would have expected to see 4.5 b with air on at 21C
the sh is slightly high at 10C though not exessive.
Discharge at 18.5 b is reaonable. As Bigfreeze suggests you could reduce the pool water flow marginally and raise the SDP to say 19.5b see if there is any effect.
Are the fans running at full speed? Are these new or existing units, have they been running ok previously? Is there any air recirculation around the evaporator causing unusually low load conditions?

[mad fridgie]

Your water temp is very low, keeping the condensing pressure low and lots of sub cooling. These are designed for 28C water. The evaps are quite small (to cover high ambient and humidity) and to keep the compressor within the working enverlope. Small drop in temp and humidity along with the low water temp will cause low suction pressures (hence freezing) Is the unit using TEV, EEV or a cap/fixed orifice?
Swimming pool heat pumps are normally not the best designed pieces of refrigeration equipment.

[nevgee]

Your water temp is very low, keeping the condensing pressure low and lots of sub cooling. These are designed for 28C water. The evaps are quite small (to cover high ambient and humidity) and to keep the compressor within the working enverlope. Small drop in temp and humidity along with the low water temp will cause low suction pressures (hence freezing) Is the unit using TEV, EEV or a cap/fixed orifice?
Swimming pool heat pumps are normally not the best designed pieces of refrigeration equipment.

I'm confused .... why are they not the best designed, do we know what the manufacture is and why do they need high air temps. 21.6C isn't a low air temperature, is it?
Why is 21C pool temperature considered to be very low? Are you saying that ashps for the pool are designed for use in hot humid climates and if so, why would they then need a heat pump?
Here in the UK ASHP for pools run quite well from the beginning of the season, when air temps can be at 0C and the start up pool temp not much better than 8C. Many run through the winter and able to achieve 28C pool temp.

Aircon50 needs to check the manufacturer's spec for the units he's working on. Perhaps they're for working in a different temp zone and not suited to where they are located.

[mad fridgie]

I'm confused .... why are they not the best designed, do we know what the manufacture is and why do they need high air temps. 21.6C isn't a low air temperature, is it?
Why is 21C pool temperature considered to be very low? Are you saying that ashps for the pool are designed for use in hot humid climates and if so, why would they then need a heat pump?
Here in the UK ASHP for pools run quite well from the beginning of the season, when air temps can be at 0C and the start up pool temp not much better than 8C. Many run through the winter and able to achieve 28C pool temp.

Aircon50 needs to check the manufacturer's spec for the units he's working on. Perhaps they're for working in a different temp zone and not suited to where they are located.
Yes you are? I did state "normally" In this case he said the evap had frost protection, not a defrost function, this indicates that it is designed to work in high ambient conditions.
21C is F*** freezing for a pool. Most heat loss from a pool is via evaporation, so wet bulb temperatures are the important factor.
If i remember correctly the base rating for swimming pool heat pumps is 24C Db 19C wb and 28C water on.
If you look at the copeland comp envelopes max SST is 15C on most refrigerants. So the evap size is critical when ambient does become higher, so high ambient models have relativlly small evaps and tend to run at a SSTof 7C at design conditions (when ambient rises above design SST rises as well, keeping within the spec) They do not tend to have CPRs, MOPs, fan speed controls and many have fixed expansion devices.
As far as the UK, well you purchase equipment that suits your enviroment, has a defrost.
i had two Calorex Heat Miser swimming pool heat pumps, each rated to 10KW UK conditions, Local suppliers (NZ and Aus) using basically the same system (comp displacement, air flow and evap surface area) would rate at 21Kw

[Gary]

TXV or cap tube?

[nevgee]

Yes you are? I did state "normally" In this case he said the evap had frost protection, not a defrost function, this indicates that it is designed to work in high ambient conditions.
21C is F*** freezing for a pool. Most heat loss from a pool is via evaporation, so wet bulb temperatures are the important factor.
If i remember correctly the base rating for swimming pool heat pumps is 24C Db 19C wb and 28C water on.
If you look at the copeland comp envelopes max SST is 15C on most refrigerants. So the evap size is critical when ambient does become higher, so high ambient models have relativlly small evaps and tend to run at a SSTof 7C at design conditions (when ambient rises above design SST rises as well, keeping within the spec) They do not tend to have CPRs, MOPs, fan speed controls and many have fixed expansion devices.
As far as the UK, well you purchase equipment that suits your enviroment, has a defrost.
i had two Calorex Heat Miser swimming pool heat pumps, each rated to 10KW UK conditions, Local suppliers (NZ and Aus) using basically the same system (comp displacement, air flow and evap surface area) would rate at 21Kw

Thanks for that ... you confirmed my query about the different temp zone. I did pick up the point about defrost and accept your comment that in the UK we do depend on defrost in most cases.

21C pool temp is cool not cold ...... maybe you've gone soft or need to swim harder. :D

[mad fridgie]

If there is no steam coming off the pool, i am not getting in it! I am One Soft ****

[nevgee]

As far as the UK, well you purchase equipment that suits your enviroment, has a defrost.
i had two Calorex Heat Miser swimming pool heat pumps, each rated to 10KW UK conditions, Local suppliers (NZ and Aus) using basically the same system (comp displacement, air flow and evap surface area) would rate at 21Kw

I note that particular make you mention has the following specs


Ambient -15°C minimum 35°C maximum
Water 10°C minimum 40°C maximum


units (ALX/BLX)
Ambient 0°C minimum 35°C maximum
Water 10°C minimum 40°C maximum

Their design control temp being 24C, Defrost becomes active requirement when the air temp drops typically down to 10 C with water on at 20C.

[aircon50]

Thank you all for your comments. However, my initial questions have still not been answered.
Is there a simple rule of thumb for SH and SC? For example, would a suitable range of SH for R407c be between 5 to 8C degs? I have tried researching from internet, but there is a lot of conflicting information! I am sure Gary will have the answers, but in Celsius please!
Back to the pool heat pump, there is no defrost cycle, just a frost sensor, and this particular unit has a TXV and is rated at 44kW. Fixed speed evaporator fan and no short cycling of discharge air.
I won't give manufacturers name, or where made, but suffice to say they are sold worldwide and therefore suitable for all temperature zones, but naturally will perform better with higher ambients. They do make models with reverse cycle, and for low ambients, particularly for N America and Canada.
Nevgee, you are right - 21C pool temp is cold!
Graham

[Gary]

I would expect SC of 5.5-8.5K and SH of 6.5-9K. You might try warming the TXV bulb in your hand. If the SH drops, then adjusting the TXV will work.

If warming the bulb doesn't drop the SH, then adjusting it won't either... in which case the TXV isn't getting solid liquid at it's inlet.

We know there is plenty of refrigerant (high SC) so I would be looking for a partial restriction in the liquid line or restricted TXV inlet screen.

[aircon50]

Thanks Gary
I knew the answers would be there somewhere!
Not had any problems in the last few weeks, but will bear in mind your comments if the problem repeats itself. Ambients here in the Canary Islands are starting to rise so hopefully no more icing!
Did have a similar problem with cap tube AWHP in January (same manufacturer), but that had been overcharged after a leak repair. Easily identified - there was 0 K SH! Recovered refrigerant bit by bit until I had approx 6K SH. No problems since.

[airefresco]

One of the heat pump models I work on regularly says in the documentation that target SH should be 7K, so I always aim for that with heat pumps. This particular range are on R407c, so is within what Gary says.

I wouldn´t worry too much about the frosting, unless excessive. I see that happening here too on machines which are working fine. You tend to see it more on machines which operate 24/7 without pool covers.