Low discharge issue

[Polar101]

Hi,

I'm working on a GS heat pump that runs on R32 but is not raising the discharge temp to where you would expect when running. All the other temps seem correct but discharge only ever is slightly over the condensing temp. Typical scenario
Brine In 8C
Brine Out 6C
Evap -1C
Superheat 7k
heating flow 35C
Heating ret 30C
Condensing 31C
Discharge 33C

I would expect the system to have a discharge temp of about 55-60C in this scenario but its not happening. Any ideas what may be the cause. EEV controller so valve regulation is pretty good.

[seanf]

Whats the EEV opening percentage?
Whats the discharge superheat?
Whats the LL sightglass look like if it has one?
Any liquid injection or oil cooling for the compressor?

[Polar101]

its a modulating compressor so the EEV opening % will vary according to the compressor speed. At low speed its about 13%, at higher speeds its closer to 30%. I've tried it with a lower refrigerant charge as I was afraid it was liquid floodback but the sightglass started flashing and the superheat was fine when it had a full sightglass so you'd imagine the evaporator is working fine. No liquid injection or oil cooling. Its a rotary compressor.

[seanf]

Is the comp speed varying on suction pressure?
Is the brine flow rate through the evap correct?
Is the water flow through the condenser correct?
Is the refrigerant charged to a labeled weight?

[frank]

If it is working and achieving temps, why the query on one aspect, which seems to be based on your expectations.
If it wasn't achieving temps then I could understand your concerns.
It reminds me of an instance when R407C first came out and I went to service a heat pump a/c. Found the outdoor coil very thick in frost/ice and thought that the defrost was faulty. Speaking with Tech support, he said "are the indoor temps ok?" which they were, then there is not a problem and it will defrost when necessary, He was spot on.
As always, it would be great to have an insight into the control strategy so we could understand what should be happening but each manufacturer keeps such information close to their chest, so to speak

[Polar101]

Is the comp speed varying on suction pressure?
Is the brine flow rate through the evap correct?
Is the water flow through the condenser correct?
Is the refrigerant charged to a labeled weight?

Compressor is varying according to distance from set point. Water flow through both condenser and evaporator are spot on. Refrigerant is charged to label weight. It's making temp but I'm worried that the low discharge is a consequence of some issue with the system which could compromise compressor lifespan.

[Polar101]

I haven't ever seen discharge temps this low before and worried about compromising the lifespan or performance. R32 is known for its high discharge temps so having such a low one is what's concerning me.

[RANGER1]

Polar101,
Can you measure superheat at compressor inlet.
Are there any heat exchangers on suction line?

[Polar101]

Yes, measured it with digital guages. It's also around 7-8k as the suction line is very short, only 3ft or so, so not much opportunity to pick up additional heat. There's no additional heat exchangers on the line.

[seanf]

Any pressure regulators, by-pass valves or 4-way valves on the system?
Assuming its just water flowing through the condenser?
What are the flow rates your seeing?
And what is the units capacity Kw?

[RANGER1]

Can you increase superheat a few degrees to see what happens?
Discharge temperature should be around 65 deg C for these conditions according to Bitzer software.

[Polar101]

Any pressure regulators, by-pass valves or 4-way valves on the system?
Assuming its just water flowing through the condenser?
What are the flow rates your seeing?
And what is the units capacity Kw?

No, no pressure regulators or valves just a very basic circuit. Flow varies with output but there's a 5k differential which indicates good flow and at peak its about 1.85m3/hr. Capacity is 10kw.

[Polar101]

Yes I'm going to try that. Checked the software for this compressor and discharge should be about 56c at those conditions. Liquid flood back is looking increasingly more likely.

[RANGER1]

Superheat can also only max out to whatever water in temperature is in evaporator, so tricky one unless maybe run lower suction pressure, if that is possible (hope my thinking is correct).

[Polar101]

Superheat can also only max out to whatever water in temperature is in evaporator, so tricky one unless maybe run lower suction pressure, if that is possible (hope my thinking is correct).

Yes, In order to achieve this the EEV will need to close, dropping the evaporation and giving you your additional superheat. Not ideal but better than replacing the compressor

[seanf]

Where abouts along the discharge pipe is the temperature being measured? and is that with your meter or the units sensor?

Are the water and brine circuits just one heat exchanger piped to another with only a pump and connecting pipes? or is there any additional exchangers or valves that might by-pass flow or introduce or remove heat?

Is the brine flow also at the 1.85m3/hr? and is that only at a 2K diff?

With steady running of the unit, is the EEV running at a steady position or varying?

[al]

If you're suction superheat is 7k, how can you have liquid flood back? Someone mentioned evap flow rate earlier, i would start here, evaporating at -1 with a brine outlet of 6 seems high, can you throttle evap flow and monitor?

[seanf]

If you're suction superheat is 7k, how can you have liquid flood back? Someone mentioned evap flow rate earlier, i would start here, evaporating at -1 with a brine outlet of 6 seems high, can you throttle evap flow and monitor?

Could the superheat being measured be a kind of averaged out reading?
With it being said theres a low discharge temp, is there a lack of heat being picked up? Could that point towards a poor heat exchange between refrigerant and the brine?

[Polar101]

Where abouts along the discharge pipe is the temperature being measured? and is that with your meter or the units sensor?

Are the water and brine circuits just one heat exchanger piped to another with only a pump and connecting pipes? or is there any additional exchangers or valves that might by-pass flow or introduce or remove heat?

Is the brine flow also at the 1.85m3/hr? and is that only at a 2K diff?

With steady running of the unit, is the EEV running at a steady position or varying?

discharge is being measured about 150mm from inlet to the condenser. One HX per condenser and evaporator. One pipe in and out, nothing to add or remove heat. Brine flow is about 2.1m3/hr at 3k. EEV is fairly static

[Polar101]

If you're suction superheat is 7k, how can you have liquid flood back? Someone mentioned evap flow rate earlier, i would start here, evaporating at -1 with a brine outlet of 6 seems high, can you throttle evap flow and monitor?

If you slow the brine flow the evaporation temp drops and the differential increasing which indicates to me that there is sufficient heat transfer going on.

[redroge]

Looking at your figures it’s running low loaded have you tried switching the comp. off to build up a load
discharge 33c heat flow 35c ?
are you controlling in heating or cooling

[seanf]

Are the flow directions of the refrigerant and brine going the correct way through the evap?
Are the flows going through the condenser the correct way as well?
What evap and condenser temps is the unit designed to give the 10Kw capacity at?
Is it just plate heat exchangers being used?

The brine inlet temps +8 Deg C, but the suction temps only at the brine outlet temp of +6 Deg C. (going with the saturation at -1 Deg C and 7K superheat).

And the EEV opening positions seem sort of low, but maybe theyve just been sized big.

.
.
.