Pilot operated evaporator pressure regulator

Abstract

The interior of the bellows is sealed at atmospheric pressure so the pressure on the outside of the bellows is resisted by the atmospheric pressure within the bellows as well as by the spring. When the pressure on the outside of the bellows (which is evaporator pressure in a refrigeration system) exceeds a predetermined amount, the bellows tends to collapse and the head of the bellows pulls away from the actuating pin and allows the spring acting on the pilot valve to open the pilot valve whereupon the pressure to the right of the head of the piston is reduced, allowing the piston to move to the right against the return spring force and thus open the outlet by moving the end of the piston wall past the slot in the stationary sleeve. When the pressure falls below the desired amount, the bellows expands and moves the head of the bellows against the actuating pin to close the pilot valve. Flow through the bleed hole in the end of the piston to the pilot valve chamber rapidly raises the pressure therein so that the return spring can move the piston to close the outlet. Even with the outlet closed, the small port in the piston sleeve allows enough refrigerant flow to insure adequate flow to the compressor to keep the compressor lubricated. The bleed hole in the piston head is sized to require substantial opening of the pilot valve which, therefore, minimizes false opening of the main valve. The 'Teflon' 'U' cup seal around the piston serves to impose a friction load and, more important, eliminates leakage flow around the piston to prevent water droplets getting between the piston and cylinder and freezing. The plastic sleeve around the cylinder 'insulates' the cylinder wall.