Decreasing evaporating pressure

[dsltung]

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?

[nike123]

Describe evaporator!

[dsltung]

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).

[nike123]

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).

Did you checked that air flow across evaporator is around 200 m^3/h per kW of refrigeration (400 fpm/ton)?

[dsltung]

The air flow accross the evaporator is only 40m^3/hr. Is that matter?

[nike123]

The air flow accross the evaporator is only 40m^3/hr. Is that matter?

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.

[dsltung]

That is a new unit, coil and filter are clean. What is the theory behind the 200M^3/h per kW of refrigeration? Thanks!

[nike123]

That is a new unit, coil and filter are clean. What is the theory behind the 200M^3/h per kW of refrigeration? Thanks!

As I recollect, that much of average humid air is needed to take 1 kW of heat with DT of about 10K. That are transport properties of humid air.

You did not said anything about nominal capacity of your system! I waiting for answer.

[dsltung]

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.

[Gary]

If I understand correctly, this unit brings in 100% outdoor air and cools it to 10C. Is this correct?

[Gary]

Did you checked that air flow across evaporator is around 200 m^3/h per kW of refrigeration (400 fpm/ton)?

If this is a 100% make-up air unit, then the above rule does not apply.

[Gary]

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.

[dsltung]

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.

[Gary]

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?

[Gary]

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.

[dsltung]

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.

[Gary]

What variable is the controller sensing?

[nike123]

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.


By your measured data you have 71,5 kW of achieved capacity, if I assume that you have misspelled 0,97m^3/h instead of 0,97m^/s and measured that amount at air exhaust of evaporator.
So, capacity is OK!

[dsltung]

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.

[dsltung]

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.

[nike123]

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.

I can only say that this dimensions to me looks small for that capacity and supposed delta T of 6K, but I may be very wrong.



The evaporator is a DX coil, copper tube Al. fin. 6 roll, 12 fin per inch, approx 800mm x 500mm in
size.

For that capacity and delta T I would expect at least 2-3m^2 and 3-4 rows

[Gary]

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.

Is the off coil temperature 10C (measured at the sensor)?

[dsltung]

Dear Nike,
Thanks, would that help if I increase the coil dimension, others remain unchange?

[dsltung]

Dear Gary,
Yes, measured at the sensor. The sensor just located 300mm downstream of the evaporator coil.

[nike123]

Dear Nike,
Thanks, would that help if I increase the coil dimension, others remain unchange?


Sorry, I wrongly interpreted that evaporator delta T is 6K. Ignore my last post.

[Gary]

I can only say that this dimensions to me looks small for that capacity and supposed delta T of 6K, but I may be very wrong.


I'm seeing an evap delta-T of 23K.

[nike123]

I'm seeing an evap delta-T of 23K.
Yep, you are right!

[nike123]

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?

Did you measured that -10°C evaporation temperature when coil is defrosted or when it is partially frosted?
What outdoor temperature is causing coil freezing? Or, what is outdoor temperature when you measured -10°C evaporation?
Did you measured airflow with instrument?

[Gary]

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.

[Peter_1]

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?

[dsltung]

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)

[nike123]

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)

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:http://farm4.static.flickr.com/3172/2882197464_b937415c80_o_d.jpg

It is pretty much easier when we have all that data to find what is wrong.

[dsltung]

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.

[nike123]

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.

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.com/forums/showthread.php?t=11903&highlight=adjust

Also, make sure that your readings at suction gauges are saturated gas because of R407 7K temperature glide.

[Gary]

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.

[dsltung]

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.



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?

[Gary]

Keep adding refrigerant until the subcooling is 8K.

Do not adjust the TXV.

Let us know what the new measurements are.

[dsltung]

I will try, and update you when I get the result.