过量供液

摘自于《2006 ASHRAE Handbook》,介绍过量供液制冷系统的原理和注意要点。

CHAPTER 1

LIQUID OVERFEED SYSTEMS

Overfeed System Operation....................................................... 1.1 Refrigerant Distribution............................................................. 1.2 Oil in System.............................................................................. 1.3 Circulating Rate......................................................................... 1.3 Pump Selection and Installation................................................ 1.4 Controls...................................................................................... 1.5Evaporator Design..................................................................... 1.6 Refrigerant Charge..................................................................... 1.6 Start-Up and Operation............................................................. 1.6 Line Sizing.................................................................................. 1.7 Low-Pressure Receiver Sizing...................................................................................... 1.7

VERFEED systems force excess liquid, either mechanically or O by gas pressure, through organized-flow evaporators, separate it from the vapor, and return it to the evaporators. Terminology

Low-pressure receiver. Sometimes referred to as an accumula-tor, this vessel acts as the separator for the mixture of vapor and liq-uid returning from the evaporators. A constant refrigerant level is usually maintained by conventional control devices.

Pumping unit. One or more mechanical pumps or gas-operated liquid circulators are arranged to pump overfeed liquid to the evap-orators. The pumping unit is located below the low-pressure re-ceiver.

Wet returns. These are connections between the evaporator out-lets and low-pressure receiver through which the mixture of vapor and overfeed liquid is drawn.

Liquid feeds. These are connections between the pumping unit outlet and evaporator inlets.

Flow control regulators. These devices regulate overfeed flow into the evaporators. They may be needle valves, fixed orifices, cal-ibrated manual regulating valves, or automatic valves designed to provide a fixed liquid rate.

Advantages and Disadvantages

The main advantages of liquid overfeed systems are high system efficiency and reduced operating expenses. These systems have lower energy cost and fewer operating hours because

?The evaporator surface is used efficiently through good refriger-ant distribution and completely wetted internal tube surfaces.?The compressors are protected. Liquid slugs resulting from fluc-tuating loads or malfunctioning controls are separated from suc-tion gas in the low-pressure receiver.

?Low-suction superheats are achieved where suction lines between the low-pressure receiver and the compressors are short. This minimizes discharge temperature, preventing lubrication break-down and minimizing condenser fouling.

?With simple controls, evaporators can be hot-gas defrosted with little disturbance to the system.

?Refrigerant feed to evaporators is unaffected by fluctuating ambi-ent and condensing conditions. Flow control regulators do not need to be adjusted after initial setting because overfeed rates are not generally critical.

?Flash gas resulting from refrigerant throttling losses is removed at the low-pressure receiver before entering the evaporators. This gas is drawn directly to the compressors and eliminated as a factor in system low-side design. It does not contribute to increased pressure drops in the evaporators or overfeed lines.?Refrigerant level controls, level indicators, refrigerant pumps, and oil drains are generally located in equipment rooms, which are under operator surveillance or computer monitoring.?Because of ideal entering suction gas conditions, compressors last longer. There is less maintenance and fewer breakdowns. The oil circulation rate to the evaporators is reduced as a result of the low compressor discharge superheat and separation at the low-pressure receiver (Scotland 1963).

?Automatic operation is convenient.

The following are possible disadvantages:

?In some cases, refrigerant charges are greater than those used in other systems.

?Higher refrigerant flow rates to and from evaporators cause liquid feed and wet return lines to be larger in diameter than high-pressure liquid and suction lines for other systems.

?Piping insulation, which is costly, is generally required on all feed and return lines to prevent condensation, frosting, or heat gain.?Installed cost may be greater, particularly for small systems or those with fewer than three evaporators.

?Operation of the pumping unit requires added expenses that are offset by the increased efficiency of the overall system.?Pumping units may require maintenance.

?Pumps sometimes have cavitation problems caused by low avail-able net positive suction pressure.

Generally, the more evaporators used, the more favorable the ini-tial costs for liquid overfeed compared to a gravity recirculated or flooded system (Scotland 1970). Liquid overfeed systems compare favorably with thermostatic valve feed systems for the same reason. For small systems, the initial cost for liquid overfeed may be higher than for direct expansion.

Ammonia Systems. Easy operation and lower maintenance are attractive features for even small ammonia systems. However, for ammonia systems operating below ?18°C evaporating temperature, some manufacturers do not supply direct-expansion evaporators be-cause of unsatisfactory refrigerant distribution and control problems.

OVERFEED SYSTEM OPERATION Mechanical Pump

Figure 1 shows a simplified pumped overfeed system in which a constant liquid level is maintained in a low-pressure receiver. A mechanical pump circulates liquid through the evaporator(s). The two-phase return mixture is separated in the low-pressure receiver. Vapor is directed to the compressor(s). Makeup refrigerant enters the low-pressure receiver by means of a refrigerant metering device.

Figure 2 shows a horizontal low-pressure receiver with a mini-mum pump pressure, two service valves in place, and a strainer on the suction side of the pump. Valves from the low-pressure receiver to the pump should be selected for minimal pressure drop. The strainer protects hermetic pumps when oil is miscible with the

The preparation of this chapter is assigned to TC 10.1, Custom Engineered

Refrigeration Systems.

Related Commercial Resources Copyright © 2006, ASHRAE

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