HVAC Liquid-Recirculating Systems
Liquid refrigerant recirculating systems are frequently fed by upward liquid flow through their evaporators. These systems are called bottomfed. This is accomplished by either mechanical or gas-displacement recirculators during the refrigerant cycle (see Fig. 10-20).
In some systems, more than a single evaporator is fed from the same recirculator, as shown in Fig. 10-20. Then, a proper distribution of liquid between evaporators must be maintained to achieve efficient operation of each evaporator. This balance is usually accomplished by the insertion of adjustable glove valves or orifices into the liquid-feeder line. Similarly, adjustment of the glove valves or insertion of orifices is also often used properly to distribute hot gas during the defrost cycle.
Equalizing orifices or glove valves are not used if the hot gas used for defrosting is fed to the bottom of the evaporators as shown in Fig. 10-20. In such cases, most of the hot gas could flow through the circuits nearest the hot-gas supply line. The same would also happen in circuits where both vertical and horizontal headers are used, as in Fig. 10-21. The more remote circuits could remain full of cold liquid. Consequently, they would not defrost.
Supplying hot gas to the top of the evaporator forces liquid refrigerant down through the evaporator and out through a reseating safety valve relief regulator into the suction-line return to the accumulator (see Fig. 10-21). Reseating safety-valve relief regulators are usually set to relieve at 60 to 80 psig to provide rapid defrost.
The use of check valves is important in flooded liquid-recirculating systems fed by mechanical gas-displacement liquid recirculators. The check valves are used where the pressure of the hot gas used for defrost is higher than the system pressure. The reseating safety-check valve must be used to stop this gas at high pressure from flowing back into the liquid supply-line.