Refrigerant DX Evaporators
For evaporators with direct expansion (DX), the refrigerant supply to the evaporator is controlled by a thermostatic or electronic expansion valve. In this system, the saturated pressure and the suction-gas outlet temperature are the control signals to the expansion valve.
The refrigerant charge into evaporators with direct expansion might sometimes be smaller than in evaporators with gravity circulation. However, this benefit is partly negated by the need for a larger heat-exchanger surface due to the lower k-value received from reserving the area for the superheat zone.
There are a few differences between the construction of a standard Vahterus heat exchanger and heat exchangers used for direct expansion. In contrast to evaporators for gravity circulation, direct expansion evaporators are arranged with the refrigerant evaporating between the plates and the secondary medium flowing on the shell side. Special attention should be paid to:
- Shell material and flow director material
- Pressure drop of the refrigerant
- Design of the distribution of the refrigerant
- The available possibilities for superheating
Benefits
- Safe solution– no ammonia leaks due to the gasket permeation
- Compact size – a separator is not strictly necessary
- Very small refrigerant charge
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