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Thread: Decreasing evaporating pressure
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21-09-2008, 01:06 PM #1
Decreasing evaporating pressure
I have a problem and would like to seek suggestion from professional. My DX PAU unit has very low evaporating temperature, the coil freeze easily when outdoor temperature drops. Design evaporating temperature is 4 deg.C but the actual gauge evaporating temperature is -10 deg.C. Coil size, expansion valve size and pipe size checked acceptable. I don't understand why the evaporating temperature goes below zero. Please advise?
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21-09-2008, 01:25 PM #2
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Re: Decreasing evaporating pressure
Describe evaporator!
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21-09-2008, 01:48 PM #3
Re: Decreasing evaporating pressure
The evaporator is a DX coil, copper tube Al. fin. 6 roll, 12 fin per inch, approx 800mm x 500mm in size. Refrigerant R407C, on coil temperature 33 deg.C RH 70% (summer condition).
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21-09-2008, 01:54 PM #4
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21-09-2008, 02:51 PM #5
Re: Decreasing evaporating pressure
The air flow accross the evaporator is only 40m^3/hr. Is that matter?
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21-09-2008, 03:01 PM #6
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Re: Decreasing evaporating pressure
How much nominal refrigeration capacity is that unit we speak about?
Of course it is ,if you have 1kW capacity, for example! That would be 5 times lower air flow then required. If that is the case, that is why your evaporation pressure is -10°C instead 4°C. Check air filters, evaporator cleanness and proper fun function.Last edited by nike123; 21-09-2008 at 03:07 PM.
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21-09-2008, 03:11 PM #7
Re: Decreasing evaporating pressure
That is a new unit, coil and filter are clean. What is the theory behind the 200M^3/h per kW of refrigeration? Thanks!
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21-09-2008, 03:19 PM #8
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Re: Decreasing evaporating pressure
Last edited by nike123; 21-09-2008 at 03:21 PM.
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21-09-2008, 03:47 PM #9
Re: Decreasing evaporating pressure
The overall cooling capacity of the unit is 70 kW, with variable refirgerant compressor. On coil temperature is 33/28 deg.C and off coil is 10 deg.C drew point. Air flow rate is 0.97 M^3/h. It is a roof top unit with 100% fresh air intake, serve an operation theatre in hospital, room design temperature 20 deg.C 55% RH. Cooling capacity meet design criteria, except evaporator temperature too low which may freeze.
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21-09-2008, 04:08 PM #10
Re: Decreasing evaporating pressure
If I understand correctly, this unit brings in 100% outdoor air and cools it to 10C. Is this correct?
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21-09-2008, 04:17 PM #11
Re: Decreasing evaporating pressure
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21-09-2008, 04:32 PM #12
Re: Decreasing evaporating pressure
On this type of unit, coil outlet superheat is critical, as is airflow. If the superheat is normal (coil fully flooded), and the coil off temperature is being maintained by dropping the saturated suction temperature, then there is too much airflow through the coil.
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21-09-2008, 04:33 PM #13
Re: Decreasing evaporating pressure
Dear Gary,
Yes, this is 100% make up air unit. I replaced a larger distributor and a larger expansion valve, the situation doesn't change.
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21-09-2008, 04:42 PM #14
Re: Decreasing evaporating pressure
Is the control system designed to maintain air off temperature/humidity?
Is it in fact maintaining that temperature/humidity?
Or is it in fact maintaining a lower temperature/humidity?Last edited by Gary; 21-09-2008 at 04:48 PM.
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21-09-2008, 04:53 PM #15
Re: Decreasing evaporating pressure
In your original post, you say that the coil is freezing when the outdoor temperature drops. This would indicate a capacity control problem. Lower than normal coil air off temp would confirm this.
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21-09-2008, 06:01 PM #16
Re: Decreasing evaporating pressure
Yes, the compressor is drived by a variable frequency controller to control the refrigerant capacity. At steady operation, the evaporation temperature still below zero. How can I improve this situation? Thanks again.
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21-09-2008, 06:12 PM #17
Re: Decreasing evaporating pressure
What variable is the controller sensing?
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21-09-2008, 06:27 PM #18
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21-09-2008, 06:34 PM #19
Re: Decreasing evaporating pressure
An off-coil temperature sensor feed signal to the controller. The controller give a 0~10V signal to the frequecy invertor which drive the compressor. The speed of compressor is varied to keep off-coil temperature at 10 deg.C.
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21-09-2008, 06:38 PM #20
Re: Decreasing evaporating pressure
Dear Nike, you are right, the air flow should be 0.97 cu.m/s. I do agree the capacity is OK, but I can't understand why the evaporator temperature below zero.
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21-09-2008, 06:56 PM #21
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Re: Decreasing evaporating pressure
Last edited by nike123; 21-09-2008 at 06:59 PM.
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21-09-2008, 07:04 PM #22
Re: Decreasing evaporating pressure
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21-09-2008, 07:05 PM #23
Re: Decreasing evaporating pressure
Dear Nike,
Thanks, would that help if I increase the coil dimension, others remain unchange?
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21-09-2008, 07:07 PM #24
Re: Decreasing evaporating pressure
Dear Gary,
Yes, measured at the sensor. The sensor just located 300mm downstream of the evaporator coil.
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21-09-2008, 07:21 PM #25
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21-09-2008, 07:32 PM #26
Re: Decreasing evaporating pressure
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21-09-2008, 07:46 PM #27
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21-09-2008, 07:56 PM #28
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Re: Decreasing evaporating pressure
Last edited by nike123; 21-09-2008 at 08:01 PM.
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21-09-2008, 07:57 PM #29
Re: Decreasing evaporating pressure
As I see it, the necessity of keeping a very low room temperature/humidity is keeping the SST (evaporating temp) abnormally low.
Two solutions would be to increase the coil size and/or decrease the airflow.Last edited by Gary; 21-09-2008 at 08:40 PM.
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22-09-2008, 06:30 AM #30
Re: Decreasing evaporating pressure
Perhaps you humiduty is lowering drastically compared to summer conditions and there's not enough latent heat in the air , so not enough load.
Haven't read all the posts...was it evaporating above 0°C at the start up? Did you checked this yourself? When was the first startup? IS something changed afterwards?It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.
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23-09-2008, 12:08 PM #31
Re: Decreasing evaporating pressure
Thanks Nike, Gary, Peter. The unit has low evaportaing temperature just few minutes after start up, and keep low at day time. Freeze only occured at night but not always. My question is even at day time the load is enough, the evaporating temperature still operates at low temperature (i.e. -10 deg.C)? When I feel the surface of the evaporating coil by hand, the temperature could be above zero, no freezing on my hand. Then why is the surface temperature of evaporator coil above zero but the pressure indicate below zero? (gauge pressure taken from the mid of coil at end elbow)
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23-09-2008, 12:48 PM #32
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Re: Decreasing evaporating pressure
If you have high superheat than you could have low saturation temperature and still relatively high heat exchanger surface temperature.
Check your superheat and subcooling.
Best thing to do when asking help here is to make full audit of system temperatures and pressures and post here.
That include following measurements:
Air temperatures and humidty, pressures at all acessible points, pipe temperatures at system points like here:
It is pretty much easier when we have all that data to find what is wrong.
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23-09-2008, 02:30 PM #33
Re: Decreasing evaporating pressure
Hi Nike, what I have is: condensing temperature 48C (20bar), subcool 4C; evaporating temperature -10C (3bar), superheat 16C; outdoor air temperature 33C 80%RH. All fresh air unit, R407C, off-coil temperature 10C.
Does it help if I increase the condensing pressure? Or change a larger evaporator coil?
Look forward to your professional advise.
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23-09-2008, 04:57 PM #34
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Re: Decreasing evaporating pressure
10°C (off air)-5K( evaporator refrigerant to air delta)-16K (superheat) = -11°C (Exactly your evaporating temperature)
If you have short pipes between evaporator and compressor, and if you measured that superheat at evaporator, you should adjust TEV superheat to 4K.
That high superheat is most likely cause of your low evaporating pressure. But before you do TEV adjustment check this whole thread:
http://www.refrigeration-engineer.co...ghlight=adjust
Also, make sure that your readings at suction gauges are saturated gas because of R407 7K temperature glide.Last edited by nike123; 23-09-2008 at 05:06 PM.
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23-09-2008, 05:31 PM #35
Re: Decreasing evaporating pressure
High superheat (16K), coupled with low subcooling (4K) indicates low refrigerant charge. Before doing anything else, add enough refrigerant to bring the subcooling up to 8K, then take all new measurements. Do NOT adjust the TXV until you have sufficient subcooling.
Last edited by Gary; 23-09-2008 at 05:42 PM.
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25-09-2008, 06:49 AM #36
Re: Decreasing evaporating pressure
I tried before, when I added more refrigerant into the circuit, coil freeze occured immediately. Before the freezing point, added more refrigerant result in increased evaporating pressure. The point is to what extend I should stop adding refrigerant. At first, I though the expansion valve is too small which reduce the refrigerant flow, so I replaced with a larger TXV, however no significant improve in evaporating pressure, can you advise?
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25-09-2008, 02:15 PM #37
Re: Decreasing evaporating pressure
Keep adding refrigerant until the subcooling is 8K.
Do not adjust the TXV.
Let us know what the new measurements are.Last edited by Gary; 25-09-2008 at 02:17 PM.
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25-09-2008, 04:03 PM #38
Re: Decreasing evaporating pressure
I will try, and update you when I get the result.
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