Ammonia

[Johnco]

Greetings. One small technical question - we have evaporator coils not working efficiently. The design criteria is 120kW at -9C. The problem we are experiencing is that the compressor is running unloaded and cycling off, would indicate under-sized evaps. The product load is still high. What we suspect is the latent load of 34kW was not added onto the capacity of the coil. Thus we could be running a coil of 86kW. Do you suggest anyone who could shed some light on this problem?

[RANGER1]

Johnco,
It seems from your description that you may not have enough ammonia in evap to make anything happen.
You could check air on/off unit temps to see if is doing much work .
Also liquid line strainer etc .
What suction pressure/temp are comps controling and pressure/temp at evaps ?
Are fans going correct way ?
How are you supplying ammonia to evaps ie tx valve liq overfeed ?

[nike123]

Measure airflow, air in temperature and humidity and air out temperature and humidity (or Wb and Db temperatures) and post here.
Then we could see if you have design capacity, or lower then required.
If you can not measure air flow, make sure that evaporator is defrosted and all fans working good and post nameplate or catalog data for air flow, but measuring is prefered.

[US Iceman]

The problem we are experiencing is that the compressor is running unloaded and cycling off.


This sounds like you do not have adequate liquid supply to the evaporators. If the cooling load was too high then the suction pressure would be higher than normal, if... you have a sufficient supply of liquid ammonia. I fthe liquid flow to the coils is too low, the suction pressure will decrease.

Are these evaporators using thermostatic expansion valves?

And,

How high are the evaporator coils installed above the compressors/receiver?

[Johnco]

Thanks for the advice, just to mention this is a new installation and we have recently changed our coil supplier / manufacturer, therefore are under the impression there could be a problem with the design / manufacture of the evaporators.

We have done the checks and balances on the system, all seems to be in order.

ON coil 0°C
OFF coil -2.8°C
Evap Temp -9°C
Clean Coil / Face Velocity 4m/s
RH on coil 85%
TD 9

[Johnco]

Measure airflow, air in temperature and humidity and air out temperature and humidity (or Wb and Db temperatures) and post here.
Then we could see if you have design capacity, or lower then required.
If you can not measure air flow, make sure that evaporator is defrosted and all fans working good and post nameplate or catalog data for air flow, but measuring is prefered.

We have done the checks and balances on the system, all seems to be in order.

These are measurements taken on site.

ON coil 0°C
OFF coil -2.8°C
Evap Temp -9°C
Clean Coil / Face Velocity 4m/s
RH on coil 85%
TD 9
Coil icing up after 4hrs of operation

Thanks for the advice, just to mention this is a new installation and we have recently changed our coil supplier / manufacturer, therefore are under the impression there could be a problem with the design / manufacture of the evaporators.

[Johnco]

We have done the checks and balances on the system, all seems to be in order.

These are measurements taken on site.

ON coil 0°C
OFF coil -2.8°C
Evap Temp -9°C
Clean Coil / Face Velocity 4m/s
RH on coil 85%
TD 9
Coil icing up after 4hrs of operation

Thanks for the advice, just to mention this is a new installation and we have recently changed our coil supplier / manufacturer, therefore are under the impression there could be a problem with the design / manufacture of the evaporators.

[nike123]

We have done the checks and balances on the system, all seems to be in order.

These are measurements taken on site.

ON coil 0°C
OFF coil -2.8°C
Evap Temp -9°C
Clean Coil / Face Velocity 4m/s
RH on coil 85%
TD 9
Coil icing up after 4hrs of operation

Thanks for the advice, just to mention this is a new installation and we have recently changed our coil supplier / manufacturer, therefore are under the impression there could be a problem with the design / manufacture of the evaporators.


Air flow please, not velocity!
Air flow you can get by measuring air speed at few places (more is better) on evaporator surface, find average value and multiply that with evaporator surface! For your case, it should be around 75000m^3/h if that evaporator has 120kW sensible heat capacity.
Also, air off humidity is required for precise calculation!

If possible, tell us model# and make!

[US Iceman]

Are these evaporators using thermostatic expansion valves?

And,

How high are the evaporator coils installed above the compressors/receiver?


It would also be helpful if you provided the manufacturers design performance data on the evaporator coils.

Capacity @ TD
volume flow of air
etc.

[Johnco]

We have measured the airflow to be 87,696m³/hr. The air-off humidity is 94%.

[nh3wizard]

It would also be helpful if you provided the manufacturers design performance data on the evaporator coils.

Capacity @ TD
volume flow of air
etc.

In order for anyone to help they need the information requested such as what type system, are tx valves used, liquid overfeed system, pumped recirculated system, so on and so on......

[nike123]

We have measured the airflow to be 87,696m³/hr. The air-off humidity is 94%.
This is what is calculated:
http://farm3.static.flickr.com/2272/2941827322_7e5b069a76_o_d.jpg

You should have -3,8°C evaporator OUT temperature to have 120kW sensible heat.
If that is ***** I would say that you check, and if necessary, adjust superheat because Tao-Te of 6,2K is high.
For Ammonia I don't have a clue what should it be, but I suppose also not more than 5K.

[RANGER1]

Product type and temp would also be interesting .
Also what happens after defrost of coil ? Does compressor load up .