System and method of hot gas defrost control for multistage cascade refrigeration system

An improved system of hot gas defrost for a multistage, cascade refrigeration system, said multistage cascade system comprised of a first stage and a second stage, said improved system of hot gas defrost function comprising:

• a first stage liquid expansion valve;
• a second stage liquid expansion valve;
• a second stage hot gas valve;
• a central heat exchanger;
• a compressor for each of the first and the second stage liquid expansion valves;
• temperature sensors;
• pressure sensors;
• a system controller comprised of a plurality of electronic components, said system controller maintaining temperature of the central heat exchanger;
• a second stage evaporator coil enclosure;
• a first stage superheat board for monitoring pressures and temperatures in the first stage, said first stage superheat board also controlling refrigerant flow in the first stage liquid expansion valve;
• and
• a second stage superheat board for monitoring pressures and temperatures in the second stage, said superheat board in the second stage also controlling refrigerant flow in the second stage liquid expansion valve, wherein the system controller shuts off the first stage compressor during the hot gas defrost, wherein the first stage superheat board fully closes the first stage liquid expansion valve to stop flow of refrigerant into the central heat exchanger so that low pressure and temperature of refrigerant in the central heat exchanger is maintained, wherein the second stage superheat board fully closes the second stage liquid expansion valve to stop flow of superheated refrigerant into the central heat exchanger so that temperature of the central heat exchanger is maintained and simultaneously opens the second stage hot gas valve which allows the refrigerant to bypass the central heat exchanger and flow directly to an evaporator coil enclosure where energy is released, wherein performance of the hot gas defrost of the second stage is consistent during the defrost regardless of external conditions.