Archive for January, 2011

HVAC Flooded recirculator

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HVAC Flooded recirculator

Bottom hot-gas feed The multiple system shown in Fig. 10-20 shows a check valve mounted in each of the liquid-refrigerant branch lines. A single solenoid valve is used in the main refrigerant line. The defrost gas is bottom fed. Top-gas feed The system illustrated in Fig. 10-21 shows a check valve mounted directly at the […]

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HVAC Liquid-Recirculating Systems

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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, […]

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HVAC Flooded floor-type blower

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HVAC Flooded floor-type blower

Gas and liquid leg shutoff Figure 10-18 illustrates a flooded floor unit suitable for operation down to ?70°F (?57°C). The gas-pressure-powered valve used in this circuit has a solenoid pilot operator. This provides positive action with gas or liquid loads at high or low temperatures and pressures. To defrost a group of evaporators without affecting […]

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HVAC Flooded-ceiling blower

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HVAC Flooded-ceiling blower

Top hot-gas feed Figure 10-16 shows a modification of the system shown in Fig. 10-15. In the system shown in Fig. 10-16, top-fed hot defrost gas forces the evaporator fluid directly to the bottom of the large surge drum. The defrost regulator (valve A), which is normally open, is de-energized during the defrost to act […]

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HVAC Flooded-ceiling evaporator—liquid leg shutoff

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HVAC Flooded-ceiling evaporator—liquid leg shutoff

Bottom hot-gas feed Figure 10-14 illustrates a flooded-ceiling evaporator. Upon initiation of the defrost sequence, the hot gas solenoid (Number 1) is opened. Gas flows to gas-powered check valve, isolating the bottom of the surge tank from the evaporator. The hot gas flows through the pan coil and the in-line check valve into the evaporator. […]

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HVAC Flooded-gas leg shutoff

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HVAC Flooded-gas leg shutoff

The system shown in Fig. 10-13 is similar to that shown in Fig. 10-12. However, the liquid leg of the evaporator dumps directly into the surge drum without a relief valve. In this system, valve C is a defrost regulator. It is placed in the suction line, where it is normally open. During defrost, valve […]

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HVAC Flooded Liquid Systems

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HVAC Flooded Liquid Systems

Figure 10-12 shows a flood-gas and liquid leg shutoff (top hot-gas feed) system. Here, the gas-powered valve is used on both ends of the evaporator. It is a gas-powered check valve. At defrost, the normally closed type-A pilot solenoid is energized. Hot-gas pressure closes the gas-powered check valves. Hot gas flows through the solenoid, globe […]

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HVAC Direct Expansion with Top Hot-Gas Feed

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HVAC Direct Expansion with Top Hot-Gas Feed

In the evaporator shown in Fig. 10-10, when the defrost cycle is initiated, the hot gas is introduced through the hot-gas solenoid valve to the manifold. It then passes through the balancing glove valve and the pan coil to a check valve that prevents liquid crossover. From the check valve, hot gas is directed to […]

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HVAC Evaporator Defrost cycle

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HVAC Evaporator Defrost cycle

When the three-position selector switch is turned to defrost, solenoid valve A and pilot-solenoid valve D close as hot-gas valve C and evaporator-pilot valve B open. This allows hot gas to enter the evaporator. Valve D now acts as a back pressure regulator, maintaining a predetermined pressure above the freezing point. After a regulated delay, […]

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HVAC Evaporator Cooling cycle

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HVAC Evaporator Cooling cycle

During the normal cooling cycle controlled by a thermostat, as room temperature rises above the high setting on the thermostat there is a need for refrigeration. Liquid solenoid (valve A) and the built-in pilot (valve D) open, allowing refrigerant to flow. The opening of the built in pilot allows the pressure to bypass the sensing […]

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