Low System Performance (Hunting ?)

[rhlg]

Hello gentle,

I’ve just joined to the group and I would like your help on the following issue.

We have a system with two evaporators, one condenser and one variable displacement compressor (VDC). Tests have shown that the total evaporators performance expected for the whole system is quite below the predicted value. The refrigerant pressure at the compressor inlet is low, which makes the compressor to understand that there is no need for cooling, causing it to stroke back. The point is that the calculated pressure drop at suction line is quite below what is being observed. Subsequent tests with fixed displacement compressor (not susceptible to suction pressure drop) have shown the same results (so the problem seems not to be the compressor). There is also temperature gradient of 10 F between the circuits of each evaporator, but I think that this may not affect the performance. There is a significant flow fluctuation at the thermal expansion valves outlet (it seems that they are fighting each other, which is normally labeled as hunting effect).

I wonder to know if you already faced this problem before and what actions could be considered to prevent the hunting effect and improve system performance?


Thanks,

[Mauro]

hello
It will be that it is not having loss of pressure in the suction line, as is mounted the copper tubing (suction) that it comes of the evaporators until the compressor? which is the frequencia that this working the compressor?

[winfred.dela]

Welcome aboard rhlg.

What is your evaporating temperature?
compressor size, evaporator sizes, etc. . ?
what control valves you have in the system?
Just need some more details to be able to visualized your system.

[rhlg]

Hi Mauro. Thanks for your feedback. Actually the system does not use copper lines. Conversely, it uses R-134 compatible refrigerant hoses in the high and low sides. The refrigerant flow coming from each evaporator is ducted in -10 size hoses and joined in a Y fitting and the total flow (evaporator 1 flow + evaporator 2 flow) is ducted in a -12 size hose and routed to the compressor. As far as I could understand, the problem seems not to be the compressor (low capacity even with the fixed displacement type).

Thanks,

[Mauro]

you, has as to measure the pressure of sucçao of the exit of the evaporator? If pressao measured will be greater that of the entrance in the compressor means that this having loss pressao





sorry, you from brazil?

[rhlg]

Hi Winfred.dela,

Here's the information :

Evaporators Capacity: 10,000 Btu/h each (Fin-tube type, 4 circuits of refrigerant)
Evaporator Temperature @ outlet: 40F
Compressor: displacement of 7 in³ ; 2000 rpm
TXV: internally equalized mechanical valve (bulb embedded to the valve body).

Let me know if more data is needed.

Thank you,

[Mauro]

you, has as to measure the pressure of sucçao of the exit of the evaporator? If pressao measured will be greater that of the entrance in the compressor means that this having loss pressao

[rhlg]

Hi Mauro,

Actually, the issue seems not to be related to the pressure drop of the line (we have increased the internal diameter of the hoses and changed the T fitting by a Y fitting and the results were the same). There is something else that is causing the problem. The fact is that the variable displacement compressor understands that there is no demand for cooling due to the low pressure at its inlet and strokes back.

Tks,

[rhlg]

Yes, I'm from Sao Paulo,

[Mauro]

Also I live in Brazil (Florianópolis), that type of equipment makes the compressor to vary the rotation? it uses transducer of pressao?

[Lc_shi]

few questions:
# one TXV for both evaporator
# evaporator in/out pressure
# condenser pressure
# suction pressure to adjudt VDC
# TXV capacity,it seem better to use externally balanced type
#power consuming of compressor
etc

regards
LC

[rhlg]

Hi Mauro,

Floripa. It's a very nice place, isn't it ? The variable displacement compressor does not change the speed. Conversely, it varies the volume displaced. (only fixed displacement compressor varies speed). The purpose of performing test with the fixed displacement one, that is not susceptible to the suction pressure drop, was to check whether the evaporators were being underfeeded. Once the performance problem persisted with the fixed one, I assumed that the compressor is not the problem.

Thanks,

[Mauro]

Hi. how vc make to modify the dislocated volume? it is through by-pass discharge for suction? Which the manufacturer and model of this compressor?

[Brian_UK]

rhlg

You say that the TEV bulb is embedded to the valve body. Do you mean that you are NOT sensing the evaporator outlet temperature with the bulb ??

See also Sporlan information on TEV with two evaporators....
http://www.sporlan.com/10-9.htm
see page 11.

[rhlg]

Hi Brian,

When I said embedded I meant that is a block type valve (Parker). It needs to measure the evaporator temperature to regulate the superheating. Thanks for the Sporlan material.

[rhlg]

Hi Mauro,

Are you from the UFSC ? I’ve a lot of friends that have graduated in the UFSC. The compressor is a Sanden SD5V10 variable displacement compressor (internally controlled variable displacement) that controls the amount of refrigerant being discharged by changing the angle of the rotating swash plate. There is internal control valve that senses the intake pressure of the refrigerant gas, which varies with the heat load in the vehicle's cabin, to maintain an optimum compressor displacement.

[Mauro]

Hi. I formed myself in FEI SBC, but I have friends in the UFSC, its compressor he is opened, he was thinking that he was air-tight, therefore work with systems of supermarkets, systems central (racks), condictionig air, metalizadoras, but already I installed varios systems of air-conditional in cabin of tractors in a industria in the city of Jaú SP.

[rhlg]

Hi LC,

Here are the answers to the questions you raised.


# one TXV for both evaporator: No, one block type TEV for each evaporator

# evaporator in/out pressure: ~35 psig/~33.5 psig

# condenser pressure: 230 psig

# suction pressure to adjudt VDC: ~ 28 psig

# TXV capacity,it seem better to use externally balanced type: 12,000 Btu/h. Block type TEV (actually it is internally and externally balanced).

#power consuming of compressor:

Sanden SD5V10 Compressor (variable displacement)
Displacement= 48.93 cm3/rev
3.6 HP Drive motor
2100 rpm

Other system data:

Subcooling=10F
Superheating=7F

Total refrigerant flow rate =6 lb/min

Thanks for your help.

[Andy]

Hi LC,

Here are the answers to the questions you raised.


# one TXV for both evaporator: No, one block type TEV for each evaporator

Is each TEV sensing bulb attached to it's own suction on the same common suction.

The sensing bulbs should be on their own separate suctions

Kind Regards Andy

[rhlg]

Hi Andy and all,

Thanks for your reply. Answering your question, each TEV sensing bulb is attached to it's own suction. Find attached a sketch of the installation.

[Lc_shi]

Hi LC,

Here are the answers to the questions you raised.


# one TXV for both evaporator: No, one block type TEV for each evaporator

# evaporator in/out pressure: ~35 psig/~33.5 psig
2.4bar~2.3bar
# condenser pressure: 230 psig
15.85bar
# suction pressure to adjudt VDC: ~ 28 psig
1.9bar
# TXV capacity,it seem better to use externally balanced type: 12,000 Btu/h. Block type TEV (actually it is internally and externally balanced).
3.5kw
#power consuming of compressor:

Sanden SD5V10 Compressor (variable displacement)
Displacement= 48.93 cm3/rev
3.6 HP Drive motor
2100 rpm

Other system data:

Subcooling=10F 5.5C
Superheating=7F 3.8C

Total refrigerant flow rate =6 lb/min
2.72kg/mim
Thanks for your help.

The data shows no problem if the refrigerant is R22,but is it design data or really checked data on site?If possible,give a sketch of your system.

regards
LC

[Lc_shi]

HI sir
I've seen your sketch which makes me understand what your blocked TXV is. Your VDC is based on your suction pressure and change the rpm. I think you should place NRV at the outlet of the two evaporators and put an accumulator to mix better.

regards
LC

[rhlg]

HI sir
I've seen your sketch which makes me understand what your blocked TXV is. Your VDC is based on your suction pressure and change the rpm. I think you should place NRV at the outlet of the two evaporators and put an accumulator to mix better.

regards
LC

Hi Lc_shi,
Thanks for your comments. It's a block TXV. The VDC does not change the rpm. Conversely, it varies the displacement volume (by changing the plate angle) in order to mantain the same suction pressure. Actually, the problem is related to hunting and we are trying to perform some tests with a X-Charge TXV to minimize this problem.

[rhlg]

It is a VCS system that uses R-134a.

[caloy_mcjan]

hi rhlg,

In your case i think the most possible cause of trouble is the expansion valve capacity or kindly observe keenly the capillary tube of power head for possible dent or punctured.

thanks

[rhlg]

Hi caloy_mcjan,

Thanks for your feedback. We have improved the routing of the hoses to minimize pressure drop, but what really made difference was the installation of liquid-cross charge TEVs that minimize the tendency for valves hunting (dampened response to suction line temperature changes). I think that minor changes in the evaporators will allow us to reach the expected capacity.

Regards,

[Renato RR]

Replace TE (internal exualisation) and put TEX (external exualisation).Change diameter of suction line obviusly is to narow.TEV is maybe oversized.Chack for bubles in liquid line.

Best regards,
Renato

[NoNickName]

If the compressor is automotive open type, then there may be a problem with the viscous coupling (I remember of a thread of an indian guy here on RE).
If not, then I think one TXV is underfeeding the other, in this case either the system is undercharged or the liquid receiver (if any at all) is too small.

Hunting may also be caused by delay between displacement variation and TXV response. After all, that is a two degree of freedom system.

[lowtempref]

Hi Rhlg,

I feel the problem is with an expansion valve due to internal equaliser. Since the readings are not available, I am presuming that the expansion valve checking the pressure of inlet to the evaporator resulting the low feed to the evaporator causing low suction pressure. Is the discharge temperature is very high? If it is so, the expansion valve with external equaliser would solve the problem. Please check. If problem is something else please update me.

[Frosty]

Hello rhlg, have you measured individual evaporator coil outlet superheats? It may even be a liquid distribution problem, measure individual circuit superheats and see what you get, you should see an average 6 deg centigrade superheat on each circuit. What about air temperature differences across the evaporators, are they equal on both coils?

Frosty

[FrozenPhiber]

Valve Hunting


One of the major problems with single ported valves is that they are susceptible to valve hunting. Valve hunting is defined as the alternate overfeeding and starving of refrigerant flow to the evaporator. This can be noticed buy the constant movement you may see in your refrigerant gauges. Hunting can be reduced buy either relocating the sensing bulb to a better location or by purchasing a valve designed for reduced hunting.

[rhlg]

Hi guys,

I would like to thank you for the very good tips you gave. As FrozenPhiber has suggested, we had to replace the current TXV by a gas-cross-charge TXV to minimize the hunting but what really made difference was the substitution of "T" fitting by a "Y" fitting. As a result of that, the pressure drop at suction line was reduced by 4 psia, allowing adequate operation of the system.

Regards,