Air Compressors

Abstract

An air compressor of the cylinder and reciprocating piston type comprises an inlet port formed in the wall of the cylinder so as to be uncovered by the piston near bottom dead center, and a spring-loaded annular discharge valve co-acting with two concentric seatings to close the upper end of the cylinder but which is engaged and lifted slightly by the piston at top dead center, the inner seating being formed on a valve guide embodying water injector means.

Description

This invention relates to air compressors of the reciprocating type, that is embodying as a basic structure a cylinder, a piston movable therein and a crank actuating said piston through a connecting rod. Volumetric efficiency in a compressor of this type is an inverse function of the clearance volume between piston and cylinder head at top dead center; it is also an inverse function of the pressure ratio. In order to achieve acceptable volumetric efficiency in compressors operating at a high pressure ratio, for example 40 to 1 or more, it is therefore essential to reduce clearance volume to a minimum.

The primary object of the present invention is to provide a simple and inexpensive high speed reciprocating type compressor of high volumetric efficiency capable of compressing atmospheric air and delivering it at a pressure in the order of forty or more atmospheres without departing from the basic structure as above defined.

A further object of the invention is to provide a high speed compressor capable of delivering a mixture of air and super-heated steam at a pressure in the order of forty atmospheres and at a temperature in the order of 300.degree.C.

A compressor according to the invention comprises a cylinder, a piston movable therein, a crank, a connecting rod driven by said crank and actuating said piston, an inlet port formed in the wall of said cylinder so as to be uncovered by the piston when near to bottom dead center, and an annular discharge valve urged by spring means towards two concentric seatings to form a closure to the end of the bore of said cylinder, the components being so dimensioned that at top dead center said piston makes contact with said annular discharge valve.

Other objects, advantages and features of the invention will be apparent from the following description and the accompanying drawings of which FIG. 1 is a sectional elevation transverse to the crankshaft center line of a compressor according to the invention, and FIG. 2 is a fragmentory view of a modification.

As shown crankshaft 1 is housed in crankcase 2 which also houses camshaft 3. Cylinder 4 is mounted on crankcase 2 and piston 5 is driven by crankshaft 1 through connecting rod 6. Cylinder 4 is provided with cylinder head 10, a ring of inlet ports 9 and a discharge port 21, and at the upper end of bore 7 of cylinder 4 is concentric seating face 8. Depending from the center of cylinder head 10 is valve guide member 11 which has flange 12 at its lower end and displaceable axially of said member with a sliding fit thereon is annular discharge valve 13 which is urged by springs 14 towards seating face 8 and concentric seating face 15 on flange 12 of valve guide member 11. Piston 5 is provided with a circular central recess 16 to clear the lower end of the valve guide member when the piston is at top dead center.

Co-axially with valve guide member 11 is water injection nozzle 17 to which water is supplied by jerk pump 18 through pipe 19. The amount of water injected per cycle is such that it leaves the compressor as super-heated steam mixed with the discharged air. Jerk pump 18 is actuated by cam 20 on camshaft 3 which is driven from crankshaft 1.

The basic components of the compressor are so dimensioned that at top dead center the crown of piston 5 makes contact with the under surface of annular discharge valve 13 thereby eliminating clearance volume between piston 5 and valve 13. To obviate the necessity of extreme refinement of dimensional control, the piston is preferably permitted to lift the annular discharge valve off its seatings by an amount not exceeding about 1 percent of piston stroke. Thus when piston 5 leaves top dead center position annular discharge valve 13 is urged by springs 14 to abut seatings 8 and 15. Further downward movement of piston 5 creates a vacuum in the space below discharge valve 13 and when piston 5 uncovers inlet ports 9 air flows into the evacuated space.

As soon as piston 5 in its upward travel closes inlet ports 9, the air trapped under discharge valve 13 begins to be compressed. When the piston has completed about 70 percent of its upward stroke, injection of water from injection nozzle 17 begins, and this injection ends before piston 5 reaches top dead center. The heat of the compressed air flashes the water spray into steam and the latent heat of evaporation of the water reduces the temperature and pressure of the air.

As piston 5 continues to travel upwards, the pressure of the air being compressed exerts on annular discharge valve 13 a force sufficient to lift it off seating faces 8 and 15; air under pressure then passes from the cylinder bore 7, across seating face 8, into the space under cylinder head 10 and out through discharge port 21.

Near top dead center the piston speed approaches zero, the rate of air flow across seating face 8 also approaches zero and annular discharge valve 13 is forced by springs 14 towards seating faces 8 and 15.

When the top surface of piston 5 at the top dead center rises about the level of seating 8, annular discharge valve 13 is held by springs 14 in contact with the crown of piston 5 and will not be able to abut seatings 8 and 15 again until piston 5 has started on its downward stroke.

It will be apparent that the only clearance volume when piston 5 is in contact with annular discharge valve 13 is the small space between the base of the recess 16 in piston 5 and the lower end of valve guide member 11. In the construction described in inner diameter of discharge valve 13 is preferably less than one-third of the diameter of cylinder bore 7 and the diameter of recess 16 in the crown of piston 5 is preferably less than 40 percent of the diameter of piston 5; with these dimensions it is possible to achieve a volumetric compression ratio greater than 500 to 1. Thus high volumetric efficiency is possible even when the pressure ration exceeds 40 to 1.

The relatively large breathing capacity provided by inlet ports 9 and annular discharge valve 13 is such as to permit high speed operation without the usual penalty in terms of volumetric efficiency.

Injection of water during the compression stroke not only decreases the work of compression, and therefore the power input requirement, but by reducing the temperature of the delivered air relieves springs 14 of undesirable thermal stress.

When compressed air is required for high pressure combustion of a hydrocarbon fuel, inclusion of water vapor is beneficial in that it is conducive to clean combustion whereby the products of combustion are relatively free of hydrocarbons, carbon monoxide and oxides of nitrogen; for such service, therefore, addition of steam to the air delivered by a compressor is not a nuisance but is desirable.

FIG. 2 illustrates an alternative construction which avoids the necessity of recessing the crown of the piston whilst retaining the high volumetric efficiency of the construction shown in FIG. 1. In this embodiment the annular discharge valve 13a has a stepped or recessed inner edge portion 13b where it seats on flange 12 of valve guide member 11, the lower end of valve guide member 11 being positioned slightly above the level of seating 8 such that using a piston 5a with a flat crown and with annular discharge valve 13a engaging seatings 8 and 15, piston 5a at top dead center engages annular discharge valve 13a but is spaced slightly from the lower face of valve guide member 11. Thus the elimination of clearance volume between piston 5a and annular discharge valve 13a is maintained, with a limited permissible lifting of the discharge valve from its seatings by the piston if desired.

Claims

I claim:

1. An air compressor comprising a cylinder, a piston reciprocable therein by crank and rod mechanism, an inlet port formed in the wall of said cylinder so as to be uncovered by said piston when near to bottom dead center, a valve guide member projecting downwards from the head of, and co-axial with, said cylinder, means disposed co-axially of said valve guide member for injecting water directly into said cylinder during the compression stroke, and an annular discharge valve urged by spring means into engagement with concentric seatings on the end of the wall of said cylinder and on said valve guide member respectively, the components being so dimensioned that at top dead center said piston makes contact with said annular discharge valve.

2. A compressor as claimed in claim 1, wherein at top dead center said piston lifts said annular discharge valve off said concentric seatings by an amount not exceeding 1 percent of the stroke of said piston.

3. An air compressor as claimed in claim 1, in which the inner diameter of said annular discharge valve is less than one-third of the bore of said cylinder.

4. An air compressor as claimed in claim 1, in which the seating on said valve guide member is formed on a flange at the lower end of said valve guide member, said annular discharge valve having a stepped or recessed inner edge portion engaging the flange on said valve guide member such that a flat-crowned piston at top dead center can engage said annular discharge valve whilst remaining slightly spaced from the lower face of said valve guide member.

5. An air compressor as claimed in claim 1, in which the seating on said valve guide member is formed on a flange at the lower end of said valve guide member, the crown of said piston being recessed centrally to clear said flange of said valve guide member.

6. An air compressor is claimed in claim 5, in which the diameter of said recess in the piston crown is less than 40 percent of the piston diameter.