spring loaded check valves containing one or more valve flow control elements
Conventional suction and discharge valves are pressure actuated, spring loaded check valves containing one or more valve flow control elements. These elements cover flow passages in the valve structure when shut, preventing back flow. When a pressure differential is established in the direction of intended flow, the pressure differential lifts the elements from the flow passage, permitting flow therethrough. Valve spring force is slight and exerts little influence on compressor valves opening.
A partial list includes: plate, poppet, reed and strip elements. Regardless of type, when the valve is closed, the area of the flow control element exposed to the upstream gas pressure is smaller than the flow control element area exposed to the downstream pressure. This is so because only that portion of the upstream side of the flow control element facing into the flow passage is in contact with the upstream gas. The remaining upstream surface area of the flow control element is in contact with the valve structure adjacent to the flow passage. This area of contact forms the seal to prevent back flow and provides mechanical support for the flow control element when an opposite pressure differential in the direction of the downstream face is present.
The large pressure imbalance present upon opening causes a high initial flow velocity through the valve. The Bernoulli flow formula predicts a velocity of thirty-eight percent of the speed of sound with a ten percent pressure drop. The almost explosive initial flow causes abrupt pressure changes within the compression chamber and adjacent gas passages that triggers the intense pressure oscillations which can be detected throughout the suction and discharge events. These pressure oscillations increase the work of compression. Also, the variations in gas density associated with the pressure oscillations and their phase angle at valve closing have an effect on the amount of gas delivered.
The whole of the downstream face is exposed to the downstream pressure when the valve is shut. Opening of these pressure actuated valves occurs when the force of the upstream gas pressure operating on the smaller upstream exposed area of the flow control element exceeds the force of the downstream pressure operating on the whole downstream area of the flow control element. The opening pressure differential of such compressor valves therefore depends primarily on the operating pressure of the compressor.
The intent of this is to reduce the intensity of the suction and discharge event pressure oscillations and thereby increase compressor efficiency and reduce capacity variations. Two embodiments of check valves meeting this objective are described. In both cases, the valves differ from the conventional compressor valve in that the area of the flow control element exposed to upstream pressure is equal to the area of the flow control element exposed to downstream pressure, immediately prior to, and at the instant of valve opening. The equal exposed area feature removes the opening pressure dependance of the compressor operating pressure.
This new orientation is achieved by sealing on the cylindrical side surfaces of the valve element. None of the sealing area is thus projected onto the upstream or downstream sides of the compressor valves element. The means of establishing an effective seal under an adverse pressure difference while permitting unhindered valve opening and flow is an important teaching of this invention. In the second embodiment, the equal area relationship before and at the instant of valve opening is achieved by the structure of the seal between the valve element and the valve seat.The frusto-conical disc spring may be flattened between the valve element and the valve seat under conditions of high back pressure, but the disc spring lifts the valve element as the pressure difference decreases.
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