Please! Sugestion for R407c

[2v6]

Hello!

I have tried to build a air-water heat pump from conventional air conditioner.
The test results are:
T out compressor: 145 F (63 C)
P out compressor: 363 psi (25 bar)
T in water : 90 F (32 C)
T out water : 127 F (53 C)

The heat exchanger is with 6 square foot (0.57 m2).
Could somebody sugest are these values OK?
Thanks in advance!

[SteinarN]

You have a high discharge pressure and high leaving water temp.

At the same time the compressor discharge temp is peculiar low. It indicates lack of suction superheat. What type of expansion device do you have?

But first of all I would increase the water flow in order to decrease the discharge pressure and thus get a better cop and probably a more long lasting compressor.

Is it for hot water heating or room heating?

[2v6]

You have a high discharge pressure and high leaving water temp.

At the same time the compressor discharge temp is peculiar low. It indicates lack of suction superheat. What type of expansion device do you have?

But first of all I would increase the water flow in order to decrease the discharge pressure and thus get a better cop and probably a more long lasting compressor.

Is it for hot water heating or room heating?

Thank you for the answer!
Sory, what is "expansion device"?
On first try the exchanger was 0.3 m2, the pressure goes very fast up to 30, and compressor works on very high load after 1 minute. Now is better. After 15-20 minutes it reaches the nominal current.
What if I put bigger exchanger - round 1 m2?

It is for floor room heating.

I will look for information about "suction superheat", but could you breafly describe what it is?

[nike123]

Thank you for the answer!
Sory, what is "expansion device"?

Try wikipedia for "Thermostatic expansion valve" and " capillary tube"

On first try the exchanger was 0.3 m2, the pressure goes very fast up to 30, and compressor works on very high load after 1 minute. Now is better. After 15-20 minutes it reaches the nominal current.
What if I put bigger exchanger - round 1 m2?

You should have about 5K temperature rise of water at heat exchanger. Correct your flow rate and exchanger size to achieve that 5K difference.

It is for floor room heating.

Floor heating doesn't need water in temperature higher than 35°C

I will look for information about "suction superheat", but could you breafly describe what it is?

What is Superheat – definition, function, setting and adjusting

Proper definition: The difference between the saturated temperature (pressure/temp chart) and the actual temperature of the suction gas.

Function: To protect the compressor! Other functions such as improving the efficiency of the evaporator are secondary. The two functions can become in conflict, however, protecting the compressor is the first priority of Superheat.

If Superheat is set to low: refrigerant may flood back, washing out oil. Leading to low oil pressure, bearing wiping and other damage may ensue. Refrigerant and or oil may slug back into the compressor resulting in broken valves, rods and other parts.

If Superheat is set to high: the compressor will get insufficient cooling. Oil breakdown and sludging, oil screen clogging, high ring wear and ultimate seizure.


Steps in setting Superheat:

1. With an accurate gauge, measure the pressure at the compressor suction service valve. Convert to temperature (°F) from the appropriate gas chart.
2. With an accurate thermometer measure the temperature of the suction line entering the compressor AT the compressor.
3. Subtract 1 from 2. The difference is Superheat in °F. Most manufacturers recommend setting at about 10-12 °F. It is essential to check the specific manufacturers recommendation.
4. Now, measure pressure at the evaporator (remote bulb location) and again convert to temperature to see if the evaporator is operating at design level. If it is, all is well in the circuit. If it is not, it may be necessary to adjust the thermostatic expansion valve. However, if this is done it is essential to remeasure at the compressor to be sure that Superheat setting is still within the manufacturers recommended range.

[2v6]

Try wikipedia for "Thermostatic expansion valve" and " capillary tube"

The outdoor unit is cheep one, therefore probably expansion is just capillary tube.

You should have about 5K temperature rise of water at heat exchanger. Correct your flow rate and exchanger size to achieve that 5K difference.

Now the water rise is about 20K. Is this means that the water flow has to be round 4 times faster?
Where have to achieve this 5K difference - between out water and in gas? Now this difference is about 10K.

Floor heating doesn't need water in temperature higher than 35°C

Yes, indeed. But there are also radiators before for faster heating.

What I see - suction superheating means high temperature inlet the compressor (a lot energy from the evaporator). In this conditions this is normal - round 8 degC outside air. Shall I have normal operation on low air temperatures (under 0)?

[nike123]

Now the water rise is about 20K. Is this means that the water flow has to be round 4 times faster?

No, water flow should be around 3l/minute per kW of heat power of condenser.

Where have to achieve this 5K difference - between out water and in gas? Now this difference is about 10K.

Between out water and in water!
Temperature difference between gas (saturated gas pressure converted to temperature and measured at discharge port of compressor) and water out should be 10K and between gas and water in should be 15K

Acceptable difference between water in and out are 4K to 8K.

You know that condenser is properly sized when at this flow (~3l/min) you got above figures.

Yes, indeed. But there are also radiators before for faster heating.

Then you should have 3way mixing valve for floor circle to keep water in temperature below 35°C
Also, when increasing water temperature your COP is lovering.

What I see - suction superheating means high temperature inlet the compressor (a lot energy from the evaporator). In this conditions this is normal - round 8 degC outside air. Shall I have normal operation on low air temperatures (under 0)?

If you have defrosting, you could expect good work to about -5°C (in relatively dry day) when you should switch totaly to backup heater.

[2v6]

No, water flow should be around 3l/minute per kW of heat power of condenser.
...

Thanks for all this information.
One other think - yesterday outside was round 15 degC and ***** goes up to 69 for less then 5 minutes.
What should be the maximum operating temperature?

[nike123]

Thanks for all this information.
One other think - yesterday outside was round 15 degC and ***** goes up to 69 for less then 5 minutes.
What should be the maximum operating temperature?

You should set high pressure switch to 28 Bar.

[2v6]

You should set high pressure switch to 28 Bar.

Hi Nike,

I have made some changes in the system.
First I have put an water meter in order to measure the flow (what was also the SteinarN's suggestion). Also open additional heat load (another room). And at the end I have change the BPHE with 1.33 m2.

So now the situation is that:
On stopped system the pressure is round 8 bar on 18 deg. When outdoor unit is heated from the sun it goes up to 10. Is this OK?

The flow now is 8.2 lit/min. It is more then twice lower then your suggestion, because the outdoor unit is 20kBTU (5.9 kW).
On round 8 degC outdoor temperature the gas goes up to 66 degC (25 degC from HE).
The water is 28 in, 37 out.
Load is 8.2 A (nominal is 8.7 A).

There is no high pressure switch, but the pressure goes up just to 18 bar! So I put high temperature switch, that switches off on 67 degC the compressor for 5 minutes for cooling.

What about the HE (1.33 m2) - is it OK?

Because of the low pressure shall I add gas?

At the moment is not possible to have more water flow. What other could be done to run better the system in this condition?

Thank you for the help!

[nike123]

Hi Nike,

I have made some changes in the system.
First I have put an water meter in order to measure the flow (what was also the SteinarN's suggestion). Also open additional heat load (another room). And at the end I have change the BPHE with 1.33 m2.

That is to big! 0,5m2 is enough!

Your old exchanger is OK!

So now the situation is that:
On stopped system the pressure is round 8 bar on 18 deg. When outdoor unit is heated from the sun it goes up to 10. Is this OK?

Yes!

The flow now is 8.2 lit/min. It is more then twice lower then your suggestion, because the outdoor unit is 20kBTU (5.9 kW).

Flow should be 18L/min

On round 8 degC outdoor temperature the gas goes up to 66 degC (25 degC from HE).
The water is 28 in, 37 out.
Load is 8.2 A (nominal is 8.7 A).

There is no high pressure switch, but the pressure goes up just to 18 bar! So I put high temperature switch, that switches off on 67 degC the compressor for 5 minutes for cooling.

Temperature response is slower than pressure response device but you could also live with that!

Because of the low pressure shall I add gas?

If there is need or there is no need for more gas, must be deducted from superheat and subcooling and not from suction pressure alone!

At the moment is not possible to have more water flow.

Water is what takes heat. You must have correct water flow or condenser is not going to be able to reject heat, and you will end with dead compressor (high compresion ratio and overheating).



This is condenser what you should have:
c2v.pdf

[nike123]

Hi Nike,


At the moment is not possible to have more water flow. What other could be done to run better the system in this condition?

Thank you for the help!

If you have problem with flow, you could use primary/secondary buffer tank to ensure correct flow.
Also, instead of radiators use fan-coils.

[2v6]

Temperature response is slower than pressure response device but you could also live with that!

Yes, indeed. On compressor change I will put pressure switch.

In the HE sellection page there is gas inlet temperature 76.7 degC. Is this normal work temperature?

[nike123]

Yes, indeed. On compressor change I will put pressure switch.

In the HE sellection page there is gas inlet temperature 76.7 degC. Is this normal work temperature?

Condenser inlet gas temperature is compressor discharge temperature - temp./pressure drop in discharge pipe between compressor and condenser.
That depends on suction temperature and suction superheat, and in good design and normal conditions could be between somewhat lower than that and max 90°C.

[2v6]

Condenser inlet gas temperature is compressor discharge temperature - temp./pressure drop in discharge pipe between compressor and condenser.
That depends on suction temperature and suction superheat, and in good design and normal conditions could be between somewhat lower than that and max 90°C.

O-o! The pipe is just 3 meters.
Is this means that I could peacifully "fry" the system up to 80 degC inlet the condenser?

[nike123]

O-o! The pipe is just 3 meters.
Is this means that I could peacifully "fry" the system up to 80 degC inlet the condenser?

Yes! ......

[2v6]

After some changes I have new mode of the system.
What could be proved?