Frozen food store a/c system problem

[friocold2008]

Hi fellows. I´m not really in the a/c business but rather in the refrigeration field. That´s why I need some advice from you the experts.
There is this small frozen food (lamb, meat cuts, seafood, turkey) store which is about 100 sq m. The products are located in several open aisle type horizontal freezers. There is a ducted central air conditioning system which is suppose to provide cooling. The problem is that whenever the a/c system is operating, the cooled a/c air plus the cold coming out of the freezers make the ambient temperature very uncomfortable for the occupants (between 18 to 19 deg C). On the other hand, if the a/c system is turned off, ambient temperature gets better, but I´ve noticed that there is an excesive accumulation of frost in the refrigeration coils, I guess due to the lack of humidity control, otherwise provided by the a/c system.
Is there anyway to get dehumidification from the a/c system without overcooling the store ?
Regards.

[Gary]

What controls the temperature in the store?

[aircon50]

Sorry if this sounds a little obvious - increase the thermostat setting for the store a/c to a more comfortable setting. Or - what about running a/c in dehumidify mode? That way the humidity/frost could be controlled, without reducing store temperature.
Regards. Graham

[friocold2008]

Thank you both for your interest, and let me explain my case a little better:
1. The central a/c system is controlled by a mechanical standard single stage thermostat.
2. Whenever the central a/c system is off, just the cold air coming out of the open freezers keeps the ambient temperature around 21 deg. C. My location is kind of a tropical one, with humidity levels normally around 60% or above, so each time a client enters or leaves there´s a lot of humidity entering the store, which ends up as frost in the evaporators. If I want to get some humidity control from the central a/c system, I have realized that I must set the room thermostat around 18-19 deg C, in order to get the system running long enough to achieve this goal. That´s when the ambient temperature gets too low for comfort.

[Gary]

Humidity removal is about coil temperature and run time. It sounds like you need to reduce the fan speed in order to both drop the coil temp (increases dehumidification) and increase the run time.

I am assuming that the A/C system has no unloaders.

If reducing the fan speed results in insufficient airflow to the far reaches of the structure, then the same effect can be had by bypassing part of the airflow around the coil.

Another option is heat pipes.

The best solution for the problem depends on the specifics of the system.

What is the temperature of the air leaving the coil?

[aircon50]

Just a thought - as the freezers are open aisle type - would it not easier, and possibly cheaper, to install either sliding, or hinged glass tops?
With regard to the climate control it is as Gary quite rightly says, it's dependent on system specifics.
Some more specific information regarding the a/c sytem, and possibly some photos of the store may enable us all to provide a solution.
Regards, Graham

[friocold2008]

ok. I´m going to try to give to you a more detailed description of the a/c system. Pictures will have to wait for a while since my camera is being repaired.
1- It is an USA made standard efficiency RUUD unit. System rated capacity is 60,000 BTUH. Runs on R-22. Air handler has a three-speed blower motor
2-Compressor is a Copeland Scroll of fixed capacity.
3-Air handler unit connected to rigid fiberglass board duct system, with square 4-way ceiling diffusers. Return air grille is located at the door that leads into the air handler room
4- Whenever the a/c system is running, return air temperature is between 18 to 19 deg F while supply air temperature (measured at the outlets) drops to 10 deg C and even lower.
5- A/C system is controlled by a single stage mechanical thermostat. I guess the differential is not adjustable since I see no adjustment controls besides the temperature setting lever and system on/off switch.
If I missed something please let me know.

[Gary]

1- It is an USA made standard efficiency RUUD unit. System rated capacity is 60,000 BTUH. Runs on R-22. Air handler has a three-speed blower motor
4- Whenever the a/c system is running, return air temperature is between 18 to 19 deg F while supply air temperature (measured at the outlets) drops to 10 deg C and even lower.


The supply air temperature (measured at the coil) should be 11K lower than the room temperature or even lower.

With the room temp at 19K, the supply air temp should be 8K or lower.

Your blower speed is much too high. It may seem counter intuitive to you, but lowering the supply air temp by dropping the blower speed will give you less cooling and more dehumidification. But keep in mind that lowering it too much will give you no cooling and a frozen coil.

[friocold2008]

Gary: I did as you suggested and set the air handler blower to low speed. We´ll see the effects tomorrow morning. Best regards