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Factory Installed Options with Roof Top Package Units

Posted by AC Contractor on June 11, 2013 at 9:45 PM

Recently we went on a commercial HVAC estimate for the Carrollwood Pharmacy on North Dale Mabry in Tampa. The initial information we had was that the unit was operating but was having humidity issues. Prior to visiting the job, we also found out that a diagnosis had already been completed and downsizing the system was recommended to solve humidity problems. Upon visiting the site, we discovered that the room being cooled in the pharmacy was a clean room and it was very small, 366 square feet. The room needed to maintain a cool temperature of 68 degrees Fahrenheit and relative humidity levels lower than 50% to avoid contamination. Because there was an existing roof curb, the owner preferred to use equipment that would fit. Unfortunately, a 3 ton system would be the smallest size that would fit the existing curb. This roof top unit would be entirely too large for the application without modifications and would be unable to control humidity.  We recommended to install hot gas reheat activated by a dehumidistat either wall mounted or built into the tstat. We also recommended to change the indoor blower motor from the standard PSC to an X-13. The other modification is to install a head pressure control on the outdoor fan motor so the unit will operate in cool weather. We will also install a freeze stat on the indoor coil to prevent freeze-up. Below are more engineering notes on how the system will operate and solve our potential client's issues.


Fan Control - To prevent air infiltration into the space, the evaporator fan will operate continuously on the lowest possible speed. The outside air volume should be set at the minimum to maintain the room under positive pressure.


Cool Mode - Provides a pre-set ratio of sensible and latent cooling effect from the evaporator coil. Because we know the equipment is oversized for the sensible load, we will design the normal cooling sensible ratio to be approximately 70%. This will be achieved through the use of an X-13 motor with 5 selectable fan speeds. These can be field adjusted to achieve the desired sensible ratio. Upon a call for cooling, the compressor shall energize and the evaporator fan shall operate at the desired fan speed. When the call for cooling is satisfied, the compressor shall de-energize and the evaporator fan shall return to minimum speed.


Dehumidification Mode - Provides a higher pre-set ratio of latent cooling from the evaporator coil. We will use the fan speed selection of the X-13 motor to achieve a 40% latent cooling ratio. In addition, to avoid over-cooling the space, we will provide hot gas reheat with an additional coil in the supply air stream. On a call for dehumidification, the compressor will energize, the evaporator fan will change to the pre-set speed, the hot gas reheat valve will open, and the condenser coil restrictor valve will close. The system shall remain in this operating mode until the call for dehumidification ends or there is a call for cooling. If there is a call for cooling, the operation shall revert to the "cooling mode" until that cooling call is satisfied. At that point, if there is still a call for dehumidification, that sequence shall resume.


Safety Controls - A condenser fan speed control shall be installed to lower the fan speed based on condenser temperature. This will allow the system to operate under low ambient conditions. In addition, a freeze stat will be installed on the evaporator coil to de-energize the compressor if the evaporator coil begins to freeze. This stat will de-energize the compressor and cause it to remain off for five minutes after the freeze stat resumes continuity. 


Contact Melo Air today to find out what indoor climate solutions are available for your business.

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