Thoughts on control logic

[US Iceman]

At the request of another member let's use this thread (or others on related tpoics) in this sub-forum.

Keep the discussion going guys...

[Josip]

Hi, US Iceman

It was good idea to start with new thread

At the request of another member let's use this thread (or others on related tpoics) in this sub-forum.

Keep the discussion going guys...

I think we have to make some criteria what we are going to discuss here.

What kind of RHVAC plant we are going to build/control here?

For example, it can be:

1.business building HVAC
2.industrial plant

-process/brewery/slaughter house/dairy
-cold store-with AC temp rooms/ +-0C (32F)/freezer rooms
-with HVAC facilities for offices

-NH3/*****s/one stage/cascade with CO2/brines/glycols
-with/without heat recovery

-capacity-small-medium-large
-compressors-depend on capacity
-condensors, evaprators
-controls

Just thoughts...

Please add other missing elements....there are a lot of them

Best regards, Josip

[cristovaol]

At the request of another member let's use this thread (or others on related tpoics) in this sub-forum.

Keep the discussion going guys...

Is there one up-to-date ice detector, in order to have a correct defrost interval in aircoolers, refrigeration or freezing application?

[US Iceman]

I am not aware of any new developments on this. Has anyone else seen anything?

[Gary]

If I had to choose a single criteria for demand defrost, it would be high evaporator delta-T (air in temp minus air out temp). This is an unambiguous indicator of insufficient airflow.

If a defrost cycle doesn't fix it, there is a defrost problem, fan problem or a dirty coil.

[Sergei]

If I had to choose a single criteria for demand defrost, it would be high evaporator delta-T (air in temp minus air out temp). This is an unambiguous indicator of insufficient airflow.

If a defrost cycle doesn't fix it, there is a defrost problem, fan problem or a dirty coil.

I wouldn't choose this criteria.
1. Suction pressure can fluctuate and delta-T will fluctuate as well even for clean coil.
2. We have two opposite factors influence on delta-T. Air flow reduction will increase delta-T and heat transfer coefficient reduction(due to frost) will reduce delta-T. Air flow function isn't linear. It is pretty complicated.

[Gary]

It has been my experience that the rise in delta-T far outweighs the insulative effect of the frost, presumably due to a (partially) compensative drop in saturated suction temperature (SST)... however, control logic needn't be limited to a single criteria. Evaporator approach (air out temp minus SST) indicates heat transfer... and evaporator TD (air in temp minus SST) indicates load.

Yes... it is complicated.

[Sergei]

Type of coil is very important. Coils with 4 fins per inch are very sensitive to the frost and air flow can be reduced dramaticly even when frost thinkness isn't significant. Coils with 3 fins per inch should have a lot of frost for significant air flow reduction.

[Gary]

Type of coil is very important. Coils with 4 fins per inch are very sensitive to the frost and air flow can be reduced dramaticly even when frost thinkness isn't significant. Coils with 3 fins per inch should have a lot of frost for significant air flow reduction.

Which might lead us to believe that defrost is needed when reduced airflow (high dT) is accompanied by a drop in heat transfer (high approach).

[Sergei]

Rate of frost formation depends of 3 factors:
1. Coil operating time.
2. Temperature difference between air and coil.
3. Relative humidity in refrigerated room.
TD, usually, is constant. Relative humidity depends of product activity and humidity of ambient air. Set point of coil operating time should be change several times per year based on ambient air humidity. This approach can give us good result which is close to the optimum.

[bill1983]

Surely the temperature control should pay attention to desired and actual superheat and subcooling as well as space temperature, airflow on and off the evaporator and condenser, fan motor speed versus airflow and compressor current. In this way wouldn't it be possible to write software to give an on demand defrost as well as highlight faults around the system?