Performance: Application of APR Control with Dehumidifying Zone

Application of APR Control with Dehumidifying Zone – Calculation Considerations

When installed in a system, the APR Control allows the application of multi-evaporator zoning. This permits the installation of many direct expansion zones; each individually controlled by a liquid line solenoid valve operated by a local thermostat and driving a single condensing unit.

Installations with one condensing unit operating as many as twenty (20) individual zones have been installed and are operating in nursing homes, motels, offices and residences.

An additional and very important feature of Multi-Evaporator Zoning is the application of one or more of the zones as a dehumidifying zone. The dehumidifying zone’s liquid line solenoid valve can be controlled by a humidistat to control relative humidity.

In a system using Multi-Evaporator Zoning, system load can be broken down as follows;

Static Load - Building and electrical loads

Dynamic Load - Sun and occupancy loads

Ventilation Load - Make-up Air and infiltration loads

Basic area zones can be sized for the Static and Dynamic loads, with the design engineer locating thermostatically controlled fan coil units strategically to maintain specified zone conditions, with or without distribution ductwork as needed.

The New Solution

Add a dehumidifying zone to specifically address the Ventilation Load. This zone can be controlled with a dehumidistat set to the desired relative humidity. Size this zone for the make-up air and infiltration load calculated for the project. Distribution ductwork can also be run to specific areas or return air plenums of the other fan coil units to satisfy ASHRAE Standard 62-1999 design criteria.

Advantages of the Special Dehumidifying Zone

When the thermostat is satisfied in standard system installations, the cooling cycle stops; but in ninety percent of the installations a circulating fan continues to operate to maintain ventilation requirements of ASHRAE standard 62-1999. This means the system is bringing in make-up air with the cooling cycle off. Relative humidity levels can rise in the conditioned space until the local area thermostats again call for cooling.

With a dehumidifying zone, the local zone fan coil units will continue to circulate air – except, all building make-up air will be brought in by the dehumidifying zone(s), where the make-up air conditions will be controlled for humidity and temperature conditions.

The APR Control makes it possible to deliver a set CFM quantity of preconditioned air to satisfy increasing ventilation requirements, independently of local zone evaporators sensible cooling needs.

This solution can be applied to a single zone systems with the addition of an evaporator coil in the outside air/make-up air duct, as well as multi-evaporator zoning applications where local area control is required.

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