Sensible Terminal Units
CoolSense – Sensible Cooling Terminals
The CoolSense HVAC design system combines a dedicated outdoor-air system (DOAS) with sensible only chilled-water cooling coil terminal units. The dedicated outdoor-air system provides the dehumidification needed to keep the spaces below a desired dew point.
Operation
Conditioned outdoor air, delivered from the dedicated outdoor-air unit, enters the terminal unit through the outside air damper. Inside the box, the conditioned outdoor air mixes with recirculated air from the zone that has passed through the sensible cooling coil. After the cool plenum air and conditioned OA are mixed, a variable-speed fan delivers this supply air through a reheat coil (if needed) and then travels downstream to the zone diffusers.
This chart depicts the operation of the terminal fan, chilled-water valve, hot-water valve or electric heater, and the ventilation air damper during a period where the zone temperature is satisfied (within the dead band) and only the ventilation requirement is needed.
In this deadband, the fan operates at its Minimum Airflow setpoint, with both the chilled-water and hot-water valves closed.
The ventilation damper is controlled to the current minimum outdoor airflow setpoint. If demand-controlled ventilation is used, this will vary between the design outdoor airflow for the zone and the minimum airflow threshold.
When the temperature in the zone rises to its cooling setpoint, (for example 75 degrees), the controller attempts to maintain zone temperature at this setpoint by modulating either the terminal fan speed or chilled-water valve. The hot-water valve, or electric heater, remains off.
When moving from this deadband to the right the controller first modulates the chilled-water valve further open, while the fan remains at Minimum Airflow setpoint. This is the 1st stage of cooling.
When the requested cooling capacity has increased to the point where the chilled-water valve is 100% open, the controller then begins to increase fan speed while the valve remains wide open. This is the 2nd stage of cooling.
The ventilation damper is still being controlled to the current minimum outdoor airflow setpoint. This means the device is bringing in the minimum ventilation airflow.
When the temperature in the zone drops to its heating setpoint, (for example 70 degrees), the controller attempts to maintain zone temperature at this setpoint by modulating the terminal fan speed and the hot-water coil or electric heater. The chilled-water valve is closed.
Moving from this deadband to the left, the controller first begins to modulate the hot-water valve further open (or increases the capacity of the SCR electric heater) to maintain zone temperature. In this 1st stage of heating, the fan continues to operate at its Minimum Fan Airflow setpoint.
When the requested heating capacity has increased to the point where the discharge air temperature reaches a user-defined maximum DAT limit (90F, in this example), the controller then begins to increase the fan speed to maintain zone temperature at its heating setpoint, while the hot-water valve (or SCR electric heater) modulates to maintain the discharge air temperature at the maximum DAT limit. This is the 2nd stage of heating.
If the requested heating capacity has increased to the point where the fan has reached its Maximum Fan Airflow setpoint, the controller will modulate the hot-water valve further open (or increase the capacity of the SCR electric heater) to maintain zone temperature at its heating setpoint, while the fan continues to operate at its Maximum Fan Airflow setpoint. (While this will cause the discharge air temperature to exceed the maximum DAT limit, it will avoid temperature control complaints to the building operator.) This is the 3rd stage of heating.
Note that in heating mode, the ventilation damper is controlled to the current minimum outdoor airflow setpoint. There is no “boost” mode for heating since the temperature of the ventilation air is typically not warmer than the zone during Occupied Mode.