Engineering practice · Air conditioning
Article 06 / 10
DX cooling coils in supply air handling units: why you cannot simply connect any outdoor unit
The idea seems simple: fit a refrigerant coil in a supply air handling unit and connect an outdoor unit. No water, pumps or separate chiller plant are needed.
This can be an appropriate solution. But a DX section in a ventilation system is not an ordinary room air conditioner with extra airflow.
The outdoor unit and coil must work as a matched pair
Cooling capacity, coil selection, refrigerant flow regulation and control logic must be coordinated. Daikin, for example, provides dedicated expansion valve and control kits for these applications. Having a kit does not remove the need to verify the specific equipment combination. 1
It is also necessary to define what is being controlled: return air temperature or supply air temperature. The manufacturer explicitly distinguishes between these options in AHU controls. 2
So the first selection question is: which parameter must this unit maintain, and under what inlet conditions?
Temperature difference alone is not enough
In summer, outdoor air can bring not only heat but also substantial moisture. If this moisture must be removed, a calculation based on temperature alone underestimates the total cooling requirement.
The enthalpy difference is used for this assessment:
where \(\dot m_{da}\) is the dry air mass flow rate in kg/s, and \(h\) is the enthalpy of moist air in kJ/kg of dry air. This approximation neglects the small correction for the enthalpy of the drained condensate. 3
For illustration, assume a flow rate of 1 kg/s, an inlet enthalpy of 70 kJ/kg and a required outlet enthalpy of 40 kJ/kg:
If the air is cooled from +30 to +16 °C, the approximate sensible component, assuming a specific heat capacity of 1,02 kJ/(kg·K), is only 14,3 kW. The remaining load in this scenario is mainly associated with moisture removal.
Selecting a 15 kW outdoor unit based on the temperature difference therefore does not cover the specified complete air treatment process.
Low-load operation must also be checked
The coil and outdoor unit interact: changes in airflow or air temperature shift the operating point of the refrigerant circuit. This is particularly important for variable-airflow systems, not just for a single summer design condition. 4
We would therefore check full-load operation, the minimum intended airflow, shoulder-season operation and start-up. We would also check whether the system can maintain stable control at low cooling demand.
Supply air temperature does not describe its moisture content
Two airstreams at +18 °C can have different moisture contents. If dehumidification is required, the relevant air condition downstream of the cooling coil must be controlled, sometimes with subsequent reheat according to an agreed arrangement. 3
The design must also provide for condensate drainage, cleaning access and coordinated responses to loss of airflow or a fault. These are part of the engineering solution, not options to address after start-up.
DX can be a compact and logical solution for ventilation. But its quality depends not on connecting an outdoor unit, but on correctly selecting the entire air treatment system.