Cooling and dehumidifying coils are freezing 1

[Buxi]

A HVAC system for conditioning of a clean manufacturing area includes a cooling system for mixed air preparation. This cooling system is used for supplied air dehumidification too. The required humidity that is set up for the manufacturing clean area is more than 35% (lower limit) and due the environmental reasons not more than 55% (upper limit).
The cooling system utilizes a two stages 40 ton (2x 20 ton compressors) condensing unit connected with refrigerant piping to two face split cooling coils in an air handling outdoor unit for mixed air preparation. All of the operations are controlled through a DDC control system.
At low loads of required refrigeration and at temperature below 50° F in the dehumidification stage, the coils are frizzing and the ice is closing 2/3 of the air pass trough surface, causing a general malfunction of the whole system. The low load control of the DX coils system seems to be through compressor unloading and hot bypass.
If you have some ideas about how this problem can be solved, please advice. A water/glycol chiller as a controlled cold source might help?
Thanks

 

[ChasBean1]

Buxi, your upper humidity limit of 55% shouldn't require you to need less than 55°F discharge air temperature from your unit (depending on indoor air temperature). Ensure the economizer cycle works right, and program the mechanical refrigeration to disable below 55°F. Also, check to see there's no bubbling in the refrigerant sight glasses (i.e., check for proper refrigerant charge). Is this a VAV system? If so, how far does the air flow reduce, and what is the square footage served by the AHU?

 

[Buxi]

Hi, ChasBean1
Thank you for the "first aid" you've already sent, I'll try to answer to your specific questions in the way I understood them; 1) Inside the AHU after the two DX refrigerant coils and gas heater follows the discharge section where the temperature is set to 57.2° F, and there is no humidity control. In this section, the pressure is set to +2.4 inch and in normal condition the supplied pressure is +2.4 inch. This pressure drops to 1.65-1.85 inch when the DX coils are freezing. 2) Following this section a reheating and humidifying section prepares the air for the clean rooms. A set point for 45% humidity controls the humidifiers. 3) The air comes in the clean room that is set to: 64.4° F, +0.10 inch (pressure) and 55% (humidity). The humidity value increases fast and turns on the DX's compressors and then the decreased value turns off the DX's compressors but not enough efficient to prevent ice building on the DX coils (outside conditions, 50° F- 85° F, and 55% - 60% humidity). Seldom the temperature (64.4° F) also turns the DX's compressors on/off. The square footage served by the AHU is 2300 sq.ft. For the same reason (DX freezing) the refrigerant was changed and no improvements were achieved.
These are a few things that might help us to improve the system, I'm kindly ask for continuing this forum, maybe we'll find a solution. Thanks again

 

[ChasBean1]

Okay. You mention that a set point of 45% humidity controls the humidifiers. I'm assuming this 45% parameter is for the discharge section of the AHU? This setpoint will only be applicable during the winter, when humidification is required.

The problem, as I see it, is three-fold:

1. 40 tons of DX cooling for 2,300 is astronomical. If this space needs that much air exchange, the whole cooling scheme will have to be re-thought. Each of the two, 20-ton stages in itself will provide too much cooling for a space that size. This should be a multiple stage application (or preferably chilled water).

2. Your AHU cools air to 57.2°F. This means that the DX coil will not dehumidify the air to below a 57.2°F dewpoint. At this dewpoint, the lowest humidity value for your room, assuming outside air is above a 57° dewpoint, is 77%. Having cooling come on to try to maintain a 55% room humidity at 64.4°F while not dropping below 57° discharge temperature will not work.

3. I would like to know the design basis for the selection of 55% as the room's humidity range. This is a pretty low value for a space maintained at 64°F. You often see limits of 55-60% applied generally to indoor air for thermal comfort and to prevent mold growth, etc., but this humidity level is applicable to standard indoor temperature conditions. Take your 72°F, 55% humid air and cool it (while maintaining the same moisture content) to 64.4°F, and the relative humidity becomes 71 percent. Sometimes well-meaning people without a full understanding of psychrometrics establish room design criteria based on faulty parameters.

If the coil can somehow consistently cool the air to 48°F during outdoor conditions above 48% dewpoint and reheat the air to control the room temperature, you'll come close to 55% humidity. There are a lot of potential solutions to the problem (including incorporating alternate forms of dehumidification) but I would be interested first to know what's driving the design parameters. Again, good luck. -CB

 

[Steviehvac]

I'm not clear on the your psychrometric parameters, design criteria etc., however dx will be near impossible to hold your conditions if entering conditions have little latent load with low entering dry bulbs. The face split coils may make the problem worse (for 100% outside air, dx coils should be fully intertwined if possible, for return air applictions face split may be preferred, it depends on application)..bottom line somwhere on the coil the evaporator temperature is below 32 F, and the entering coil dew point is above 32 F which causes freezing.

Heres a suggestion...you mentioned it before...add a chilled water coil to the air flow path (somewhere up stream of the reheat). Since the Dx is freezing at low loads, Determine chilled water coil capacity to provide the needed dehumidification at low loads only (set your ddc to keep the dx disabled at these conditions). Hopefully your required supply air dewpoint is not below 45 F (otherwise you may be looking at desiccant dehumidifiction).


Use a discharge air controller with analog output to control the chilled water valve. The valve will modulate to hold discharge air setpoint. It the load increases beyond the chiller capactiy, enable the 1st stage of dx coil step control. As long as each step of dx coil capacity is less than the chilled water coil capcity, the chilled water valve should modulate the system capacity and hold a fairly steady set point.

I did this retrofit several years ago...an electronics application. We used all the DX capacity and acheived good Design, part load performance. We were lucky, we had an existing chiller and adequate capacity. plus we had physical space for the added coil/drain pan and Fan BHP. We used SCR controlled elec heaters set for a constant leaving air of 68F. Unit operated down to 55 F dry bulb entering the coil (our goal was 48 to 50 F dewpoint, but because of dewpoint control limitations we ignored conditions below 55 F dry bulb)

by the way, it was a job sold to the client without any engineering evaluation (just a sales rep and an inexperienced contractor)...the plant engineer couldn't explain the specs for the clean room and no-one knew much about dewpoint...amazing $60,000 worth of the wrong equipment for the job...I hope your not in the same situation.

good luck