Heat Pumps at Low Ambients

[Pingo]

Hi everyone

Maybe someone can enlighten me ?

I am working on a design for a customers Gym area.

The heat pump (Daikin wall mounted) will mainly be used for heating the area but I see that the Daikin performance figures for heating is based on a nominal heating figure of: ambient temp.= 7c db 6c db
internal temp. = 20c

How much if any does the heating output drop when the ambient drops to 0c or -3c ?

Do I have to contact Daikin Technical office on this, or is there a common formula which I can use which will shows % output drop in Kw for every drop in 1c ambient temperature.


Regards Pingo

[VRVIII]

Hi everyone

Maybe someone can enlighten me ?

I am working on a design for a customers Gym area.

The heat pump (Daikin wall mounted) will mainly be used for heating the area but I see that the Daikin performance figures for heating is based on a nominal heating figure of: ambient temp.= 7c db 6c db
internal temp. = 20c

How much if any does the heating output drop when the ambient drops to 0c or -3c ?

Do I have to contact Daikin Technical office on this, or is there a common formula which I can use which will shows % output drop in Kw for every drop in 1c ambient temperature.


Regards Pingo

Hi Pingo,
If you go to Daikin extranet and download the technical data book for the unit in question, this contains a capacity table giving the capacity at lower ambient conditions.
I have attached the heating capacity table for an RZQ125D as an example.
Also remember to factor in the pipe run length correction factor, although this will have a far greater effect on cooling mode capacity.
RZQ125-DV1_heat cap table.pdf

[desA]

You may also want to check the unit operating COP & compressor discharge temperature when the outdoor air temp is very low.

The COP will reduce dramatically, & be very careful of the compressor discharge temp exceeding manufacturer's operating specs.

[VRVIII]

You may also want to check the unit operating COP & compressor discharge temperature when the outdoor air temp is very low.

The COP will reduce dramatically, & be very careful of the compressor discharge temp exceeding manufacturer's operating specs.

Hello Desa,

You’ve got me curious, why do you think the discharge temp would be problem? Providing the unit is operated within the manufactures ambient/indoor operating range?

[desA]

Hello Desa,

You’ve got me curious, why do you think the discharge temp would be problem? Providing the unit is operated within the manufactures ambient/indoor operating range?

Well, try it for yourself. Take a set of readings of a unit you know well, under very low outdoor ambient conditions - make sure to measure the compressor discharge temp.

Then go to the performance chart for that particular compressor & see how close it lies to the edge of the operational envelope. You may get a shock to find out how marginal the operating condition is.

Sidebar:
I research, design & make AWHP's for a living...

[VRVIII]

Well, try it for yourself. Take a set of readings of a unit you know well, under very low outdoor ambient conditions - make sure to measure the compressor discharge temp.

Then go to the performance chart for that particular compressor & see how close it lies to the edge of the operational envelope. You may get a shock to find out how marginal the operating condition is.

Sidebar:
I research, design & make AWHP's for a living...

Interesting, but when thinking about it this is when the discharge temp limiting function would come in to play on inverter unts (reducing compressor freq/speed) or the liquid injection would be enabled on some units.
But these would also have an effect on the overall system capacity.

[desA]

Interesting, but when thinking about it this when the discharge temp limiting function would come in to play on inverter unts (reducing compressor freq/speed) or the liquid injection would be enabled on some units.
But these would also have an effect on the overall system capacity.

What essentially happens is that, with lower outdoor temps & a fixed delivery temp, the compressor pressure ratio rises. This rise in Pr translates into a rise in compressor discharge temp - which, can be fairly dramatic, at times.

The compressor manufacturers limit the continuous discharge operating temp to around 107'C - this forms the cut-off point on the envelope.

I'd be interested to see how the variable freq/speed systems cope with this problem. If they reduce frequency/speed, then this would impact performance delivery & COP.

[Thermatech]

Most of the VRV / VRF systems have some form of discharge high temperature protection & monitor compressor discharge temp & deploy some form of discharge temperature rise protection control stratagy. This involves slightly reducing compressor speed.
If the discharge temp continues to rise above a preset maximum then the system will stop on a preliminary protection & if the system stops 3 times within a set period of time the system will lock out on a discharge over temperature fault.
For example Mitsubishi City Multi R410a VRF
max discharge temp >115 deg C = comp slows by 5 hz every 30 seconds untill below 115.
At 120 deg C system stops on preliminary fault & automatically restarts after 3 min.
If system stops on prelim 3 times in 30 mins system locks out on discharge temp fault.

I think you will find that Daikin & Hitachi do much the same with reduced comp speed but have slightly different control stratagy to compensate for some other things such as outdoor air temp or suction SH.

Generally I find the discharge temp will run lower than 100 for heating operation even with heavily frosted outdoor coil. In fact if it is running in the 90's then I am looking carefully at the operating data to see if there is a problem. If the discharge temp gets high enough for discharge rise protection to be activated then there will be some problem / malfuction in the system.

For larger VRF systems the total pipe length can easily make 15% reduction from nominal quoted heating capacity.
Low ambient defrosting can easily make another 15% reduction in overall heating capacity because the unit is not heating for 5 to 10 mins every hr due to defrosting operation.
Many manufacturers quote nominal heating duty at just above ambient defrost conditions of around 6 to 7 deg C WB.
But if in reality you need the heating duty at minus 5 then you can easily be looking at another reduction of arround 20% from the quoted nominal heating capacity.

Many manufacturers provide the correction factors in graph format in the product data technical manuals but you would be amazed at the number of consultants & contractors who sellect the equipment on the nominal capacity but forget to make any correction for pipe lenght + defrost + low ambient.

[desA]

^ Thank you very much. That is excellent information.

Would you perhaps be able to indicate where the discharge temperature measurement point is relative to the compressor discharge? This would be incredibly helpful.

The temp limits of 115'C & 120'C are very interesting to me. I'm amazed that they're allowing it to go so high - perhaps it's because of intermittent operation? Copeland typically cries foul at 107.2'C, measured 150mm from the compressor discharge nozzle.

The numbers I was citing really apply to an air-to-water heat-pump, where the lift is much higher than air-to-air. The same principle would however apply to deeper outdoor temps - should they occur, without trip activation.

Thanks again.

[Thermatech]

Have never actually measured but guess discharge temp sensor normally within 200mm from compressor discharge port.

For VRV & VRF the manufacturers use there own compressors.

Hitachi is an interesting one as it uses a high pressure scroll which discharges the hot gas into the compressor shell & the shell sort of acts as an oil sepperator.
The hot gas flows arround the motor windings so the motor windings are not cooled by the suction gas but are adequately cooled by the discharge gas.
The discharge sensor is fixed to the compressor shell & safety trip at 132 deg C if it stays at this temp for 10 mins.
The system does slow the compressor speed first with a high discharge temp protect stratagy.

[desA]

^ Many thanks...

[VRVIII]

Hi Thermatech/desA ,
Daikin VRV units also use a similar strategy to protect against high discharge temperatures, but they also monitor discharge superheat as well as discharge pipe temperature. They have been using high pressure shell scroll compressors since VRV2 was lunched (all 410a VRV models).

Discharge pipe temp control:
Discharge temp >105 C or discharge superheat >55 C = subcooler EEV target superheat will be reduced to 0 C allowing a small amount of liquid refrigerant to enter suction line (similar to liquid injection).

Discharge temp >135 C or discharge temp minus ambient temp = >115 C = Reduce compressor capacity by 1 step
Dt >135 C or Dt-amb > 115 C for 30 sec = Continue reducing compressor capacity
Dt >130 C or Dt-amb >120 C for 90 sec= Inv comp 52Hz std off
Dt > 135 C or Dt-amb >120 C for 10min = unit is stopped, if this reoccurs 3 times in 100 minutes unit will be locked off and F3 displayed at controllers

desA, Thinking about what you said make sense as the compression ratio must increase for the unit to maintain a target Tc but at the same time reducing Te to allow heat to be absorbed at lower ambient temperatures.
The max discharge temp allowed does appear to be bit higher than 107 C you referred to, but this is only allowed for 10min max. As Thermatech said they normally operate with dt of 85~100, anything higher than 105 I would expect to find a problem with the unit.

[desA]

Thanks so much for this detailed information. It is extremely useful.

They seem to be managing a transient (temporary) discharge over-temp in a smart way. I like the idea of also managing the discharge dT... that is new to me.

It's given me a lot to think about. Thanks VRVIII & Thermatech - much obliged.

[ginger prince]

Most manufacturers data books and catalogues give the capacity at EN14511 (nominal) conditions which are 21C indoor and 6C outdoor temp.

As a really rough guide you can asssume that if a unit gives 10kW of heating at 6C outdoor (nominal), you can assume a 17% drop at 0C so thats 8.3kW, a 30% drop at -5C so thats 7kW and a 45% drop in heating capacity at -10C so thats 5.5kW.

but as stated above its best to check in the data books for real figures.

good luck