The flooded evaporator, shown in Fig. 6.9, relies on natural convection to circulate more refrigerant through the evaporator than what evaporates. All inside surfaces of the evaporator are thus wetted with liquid refrigerant. The vapor formed in the evaporator is separated in the surge drum and flows to the suction line.
A level-control valve admits liquid refrigerant to replace the amount vaporized. The difference in static pressure in the liquid leg is greater than that of the mixture of vapor and liquid in the evaporator tubes, and this difference
in pressure is the motivation for the flow of refrigerant.
Several advantages of flooded evaporators in comparison to direct expansion are:
– the evaporator surfaces are used more effectively because they are completely wetted
– problems in distributing refrigerant in parallel-circuit evaporators are less severe
– saturated vapor rather than superheated vapor enters the suction line, so the temperature of suction gas entering the compressor is likely to be lower, which also reduces the discharge temperature from the compressor.
Several disadvantages of the flooded evaporator in comparison to the directexpansion evaporator are:
– the first cost is higher
– more refrigerant is needed to fill the evaporator and surge drum
– oil is likely to accumulate in the surge drum and evaporator and must be periodically or continuously removed.
The configuration of the flooded coil shown in Fig. 6.10 where the tubes of a circuit are arranged in a vertical plane exists in some applications, such as in ice thermal storage installations. Here multiple parallel circuits are located side by side each feeding from and returning to the surge drum or header pipes. The vertical circuit works well for moderate evaporating temperatures, but the height of the liquid leg becomes more of a consideration for low-temperature evaporators. Because the static head of the liquid leg results in a higher pressure at the bottom of the leg, the evaporating temperature is higher at the lower section of the evaporator, which reduces the heat-transfer rate. Table 6.3 shows penalties in the evaporating temperature per unit length of static head for R-22 and ammonia at two different evaporating temperatures. Because liquid R-22 is more dense than liquid ammonia, the penalty for a given magnitude of liquid head is greater for R-22.
In low-temperature evaporators it becomes more crucial to keep the static head at a minimum. For this reason, evaporator coils are often constructed as shown in Fig. 6.11, where the evaporating sections of the circuits rise, as
necessary, but on an incline. The compromise that emerges is that the p that motivates the flow is also proportional to the difference in elevation between the liquid in the surge drum and that of the bottom of the tubes.
Associated with this compromise is how much elevation of the surge drum above the coil to provide. Often the coil is installed beneath the ceiling and a large vertical distance between the surge drum and the coil poses headroom problems. A survey of some coil manufacturers revealed that typical distances from the bottom of the surge drum to coil to’ be between 0.15 and 0.25 m (6 to 10 in). When physical restrictions force other arrangements, such as placing the surge drum above the roof in order to conserve headroom in the refrigerated space, this information should be communicated with the coil manufacturer.
One coil manufacturer4 bases its design on circulating twice as much liquid as evaporates and on a coil pressure drop of approximately 1.4 kPa (0.2 psi) and arrives at a height from the liquid level in the surge drum to the top of the coil of approximately 0.46 m (1.5 ft).