At ESC we use our background in HVAC and Refrigeration to design solutions for non typical applications. This involves careful selection of all of the components from the refrigerant selection, tubing size, number of rows and fin spacing of the coil, the metering devices, condensing unit selection, and pipe sizing and configuration. Used in harmony with electronic controls, we can dehumidify and/or modulate the refrigerant flow to maintain precise conditions
Each component can add complexity to a DX cooling system. Each one of the complexities requires extra controls, control strategies and refrigeration components. Please see the options below.
Head Pressure Control
Fan Speed Controller or Flooded Condenser Controls.
The condenser has to be sized for the heat of rejection during the summer conditions. During colder outdoor temperatures the condenser is oversized and low head pressure results. This will cause erratic operation of the expansion valve. Some method of head pressure control is required. The selection of which method is dependent on how cold the ambient conditions are and what reliability is required. The two commonly used methods are condenser fan speed control and flooded condenser control.
The condenser fan speed control can provide reliable operation when the outdoor temperature is as low as -29 deg C. The components required are, a condenser fan motor designed for variable speeds to be used in conjunction with a solid state fan-speed control and a refrigerant transducer. The transducer senses the head pressure of the compressor and the fan speed control varies the speed of the fan to maintain a constant condensing temperature. On a multi fan condenser the additional fan motors are controlled by ambient thermostats.
The flooded condenser control can provide reliable operation when the outdoor temperature is as low as
-34 deg C. The components required are, heated and insulated receivers and a head pressure control valve. The heated and insulated receivers maintain sufficient pressure for starting at -34 deg C. The head pressure control valve restricts the flow of refrigerant from the condenser in the cold weather. This causes the refrigerant to back up in the condenser which reduces the effective condenser surface. As the head pressure control valve is restricting flow from the condenser it is also bypassing hot-gas into the receiver insuring proper head pressure is maintained.
Dehumidification Control
Dehumidification usually requires a deeper evaporator coil in the direction of airflow and may require a colder coil temperature. A cooling coil has to bring the air stream temperature down to the dew point before it removes any moisture. As cooling and dehumidification loads vary a problem develops when the cooling load is low and the dehumidification load is high. The compressor has to run to dehumidify which requires reheating of the air to maintain room temperature. This reheat can be provided by an electric heater, steam or hot water coil or condenser reheat coil. The relative humidity required in the condition space may require a lower coil temperature than standard air conditioning. When this is required, measures to ensure that the evaporator coil does not frost up, must be taken. This is best done with an electronic hot gas valve and a sensor mounted at the coil to maintain a minimum coil temperature.