DOE/ORNL Heat Pump Design Model

Notes on Dehumidifier and Outdoor Ventilation Air Options

Dehumidifier Option

The new dehumidifier option directs the air-side output of the evaporator to the inlet of the condenser. By default, the condenser airflow is set equal to that of the evaporator in the Web input interface. Because of this, inlet temperature and humidity to the condenser cannot be specified as input in the dehumidifier mode.

If the condenser airflow is instead specified to be more than that for the evaporator, the difference is assumed to be additional air drawn into the condenser from the indoor ambient at the evaporator inlet temperature. This bypass airflow is then mixed with the evaporator exit air to define the inlet conditions to the condenser. (This is sometimes done in standalone dehumidifier designs to keep the condenser inlet temperature from dropping too low.)

If the condenser airflow is specified as less than that of the evaporator, it will be reset to the evaporator airflow.

It is recommended that the fan power for the dehumidifier be modeled as part of the condenser input with the evaporator fan power set to zero. In fact, for a dehumidifier case, the evaporator fan power is set to zero by default and the usual nominal indoor fan power of 365W/1000 cfm is instead used as the default for the condenser fan. The air-side pressure drop for the condenser fan is calculated for the dehumidifier mode to be the sum of the evaporator and condenser coil and evaporator system pressure drops.

The output in the dehumidifier mode has been modified to provide performance definitions commonly used for dehumidifiers. This includes Energy Factor, moisture removal rate, and rejected heat to space in the energy summary page and in the parametric output.

The parametric setup has also been changed to select indoor RH as the default parametric. This lets one generate a set of data for a range of inlet RH levels. Alternatively, one can run a two-variable parametric of indoor ambient and RH to generate a performance map for the dehumidifier over a range of DB and RH conditions.

The airflow parametrics for the dehumidifier case have also been setup to keep the ratio of the evaporator to condenser airflow constant as either airflow is varied. (Note that this means that parametrics of both airflows at the same time not vary the airflows independently and so this choice is not recommended.)

Ventilation Option

The second new option is an outdoor air ventilation mode. This enables the introduction of outside ambient air into the indoor unit (or evaporator in the case of the DH option) to be mixed with the return air from the house. This option is available for cooling, heating, or dehumidifier modes.

When ventilation air is selected, the outdoor ambient conditions are specified as well in place of specifying them on the outdoor coil page (or condenser page for a DH). Note that when ventilation air is specified, the input indoor (or evaporator for DH) airflow is taken to be the sum of the ventilation and return airflows.

The ventilation airflow can be specified directly or given as a fraction of the specified total coil airflow. Fan power for a ventilation fan, if used, can be specified as well. It is added to the evaporator fan power in the model.

If one wishes to supply a dehumidifier with 100% ventilation air, this can be done by setting the ventilation airflow equal to the evaporator inlet airflow or using a ventilation airflow fraction of 1.

If needed, the fan power of a separate ventilation fan can be specified along with the flow rate.