GB2054101A

GB2054101A - Differential pressure valve

Info

Publication number: GB2054101A
Application number: GB7924775A
Authority: GB
Original assignee: PRESTCOLD Ltd
Current assignee: PRESTCOLD Ltd
Priority date: 1979-07-17
Filing date: 1979-07-17
Publication date: 1981-02-11
Also published as: DE3026989C2; DE3026989A1

Abstract

The valve is associated with, for example, an oil pump for a refrigeration compressor, and senses whether the pump is delivering a desired minimum pressure. The valve has inlet and outlet ports 1a and 2 in communication with inlet and discharge sides of the pump respectively. A valve member 3 is biassed by a spring 5 into contact with the inlet port 1a and blocks it. When the pump is delivering fluid at a first predetermined pressure drop across the pump, the valve member 3 is moved from its seat. A bleed flow now passes the valve member 3 passage 4 or by virtue of a slight clearance it has in the valve chamber 10. This flow impinges on a self heating thermistor, which is thus cooled to a temperature which indicates whether the required pressure difference is being supplied. When the pressure difference approaches a second, excessive, value, the valve member 3 uncovers a pressure relief outlet port. and the pressure difference cannot build up further. If desired, the sensor 7 can also respond to the temperature of the fluid being pumped. <IMAGE>

Description

SPECIFICATION Differential pressure valve The invention relates to differential pressure valves.

The invention provides a'differential pressure valve, which comprises a valve member movable to allow a bleed flow to pass from an inlet port to an outlet port when the pressure difference across the valve member reaches a first predetermined value, and a sensor for sensing the bleed flow through the valve. The differential pressure valve according to the invention senses whether the pressure difference across the valve member, and hence between the inlet and outlet ports, does or does not, reach a desired minimum. The differential pressure valve may for example be associated with a pump, the inlet being in communication with the discharge side of the pump and the outlet being in communication with (or at least at the same pressure as) the suction side of the pump. The sensor will then sense whether the pump is delivering a desired minimum pressure.Previously, a typical device for performing this function inluded a bellows arrangement with the high and low pressures in communication with each side. Such a device was relatively expensive and relatively unreliable.

The bleed flow may be provided by a narrow bore extending from one end to the other of the valve member, or by means of a narrow clearance between the sides of the valve member and the side wall of the valve chamber.

Advantageously, the valve member is further movable to uncover a pressure relief outlet port when the pressure difference across the valve member reaches a second predetermined value.

With this arrangement, one component (the differential pressure valve) performs the function of the two that would have been needed hitherto, namely, a differential pressure control for sensing when the pressure reaches a desired minimum, and a pressure relief valve for preventing the pressure exceeding a desired maximum.

Advantayeously, the valve member is movable along an elongate valve chamber against the force of biassing means, the inlet port being in one end of the chamber against which the valve member can seat, and the relief outlet port being in the side wall of the chamber spaced apart from the inlet port. The spacing of the relief outlet port from the end of the chamber in which the inlet port is located is chosen in relation to the characteristics of the biassing means and to the valve geometry such that the valve member lifts off the seat closing the inlet port when the first predetermined pressure difference is reached, and retreats as far as the relief outlet port when the second predetermined pressure difference is reached. The valve member is preferably cylindrical. The biassing means is preferably a helical spring.The bleed flow outlet and the pressure relief outlet may be the same outlet.

The sensor may be of indirectly heated or selfheating type that changes its resistance when exposed to a change in heat dissipation rate, (due to the resulting change in its temperature) or of the type that changes its capacitance or resistance depending on whether the working fluid bridges two conductors or not. The sensor is preferably a self-heating or indirectly heated thermistor. When the bleed flow takes place, the sensor is cooled. The sensor may be such that it can sense when the temperature of the fluid being pumped exceeds a certain level. This provides an additional warning; for example, if the fluid is oil, it may be desirable to cease operation if the oil temperature becomes excessive even though a sufficient oil pressure is generated. The sensor may be located at the end of the valve chamber remote from the inlet port.

The differential pressure valve may be incorporated into the body of the pump, so that the valve chamber is defined by the body of the pump, or it may be a separate unit insertable into a pump. The pump may be for a compressor, such as a refrigeration compressor.

A refrigeration compressor, the oil pump of which has a differential pressure valve constructed in accordance with the invention, will now be described, by way of example, with reference to the accompanying drawings, in which; Figure 1 is an end view of the oil pump; Figure 2 is a side view of the oil pump; Figure 3 is an axial cross-section of the differential pressure valve when closed; and Figure 4 is an axial cross-section of the differential pressure valve when open.

Referring to Figures 1 and 2, the oil pump for the compressor (not shown) is of the rotary type. It is secured to the crankcase and driven by the crankshaft via the key 1. The left hand side (as seen in Figure 2) of the pump is thus open to the interior of the crankcase, which is pressurised.

Referring to Figure 3, the differential pressure valve comprises an inlet port 1 a, a pressure relief outlet port 2, a sensor in the form of a self-heating thermistor 7, an outwardly threaded body 8 bearing the thermistor, a valve chamber 10 formed in the body of the pump, a valve member in the form of a plunger 3, and a spring 5 which biasses the valve into the closed position. The oil inlet port 1 a is connected via a passageway 1 2 to the discharge channel 13 of the oil pump (which forces oil along the centre of the crankshaft), and the pressure relief port 2 opens into the left hand side of the pump (as seen in figure 2).

The pump receives oil on its suction side via port 1 5 on the same side, and thus the suction port of the oil pump, and the pressure relief port are both at the pressure obtaining within the crankcase.

Referring to Figure 3, inlet port 1 a connects with the high pressure discharge side of the oil pump and under normal starting conditions of the compressor, oil from the pump discharge exerts a force on the bottom of the plunger 3, which is counteracted by the spring 5 and the force created by the pressure above the plunger 3 until the designed opening minimum pressure difference across the plunger is reached.

The chamber 10 is vented back to the oil reservoir supplying the suction side of the oil pump via passage-ways 2 and 6 so that the plunger 3 areas exposed to the high and low pressure sides of the lubrication system and the force exerted on the plunger 3 in the closed position by the spring 5 can be selected to give the minimum differential pressure required to open the valve. Alternatively, this setting may be made adjustable by making the closed spring length manually adjustable by some means such as a screw (not shown).

Directly the minimum differential pressure setting is attained, the plunger 3 is forced off its seat 11 and allows a small quantity of lubricating oil to pass through passage-way 1 a and through narrow bores 4 in the plunger 3 under pressure, so that a small jet of the oil impinges on the sensor 7.

In practice, it has been found that passage-way 4 can be disposed with if the clearance between plunger and walls of chamber 10, together with the position of the sensor are designed so that oil passing through this gap is brought into contact with the sensor 7. It is to be noted that the flow across the plunger 3 is a bleed flow only and not sufficient to affect the operation of the pump. It should be less than 1% of the output flow.

The larger area of plunger 3 operated on by the high pressure fluid directly it leaves is seat 11, ensures that it opens without chatter against its seat and remains open unless the oil differential pressure falls below the minimum opening pressure. This closing differential pressure is selected to be the minimum for which the compressor will run without damage to bearings or bearing surfaces.

Under normal operating conditions, the sensor 7 is bathed in oil and via its associated electronic control unit permits the compressor to run with no warning indicators on.

Should the oil pressure fall to the point where the differential pressure across the plunger 3 allows it to return to its seat 1 the oil flowing over the sensor 7 is stopped and any above the plunger 3 drains back to the low pressure area via passage way 6. Here again chatter is avoided by the smaller area of the plunger subjected to the high pressure side. Because of the sensor 7 being no longer cooled or in contact with oil, its value of resistance is changed, which activates the electronic control circuit to cut-off the power supply to the compressor and/or to give a warning indication that some form of lubrication failure is occurring.

Oil pressure may not build up due to refrigerant absorbed in it for some minutes after start-up and the compressor design is such as to allow for this without damage to its motion work.

In these circumstances a time delay is required to stop the differential oil switch from nuisance tripping during this time period. This time delay is preferably but not necessarily incorporated in the electronic control.

Referring to Figure 4, on the differential pressure rising to a maximum allowable Figure, the plunger 3 rises further in the chamber 10 against the increasing sprin'g force to uncover port 2 which allows high and low pressure areas to be directly connected to by-pass the fluid via inlet passage-way 1 a, chamber 10 below the plunger and passage-way 2, the spring characteristics and height of port 2 being selected so that port 2 is just fully open when the maximum pressure differential is reached.

The sensor is of the self-heating thermistor type with its resistance dependant on its temperature.

Its switching point in conjunction with the electronic control is selected so that it will also give protection or warnings against excess temperatures of the oil. Thus the compressor will be switched off, and/or warnings operated, if the oil temperature becomes excessive even though the pressure falls within the desired limits.

If desired, additional circuitry may be provided for other control devices such as motor over-load protection, temperature controls and fluid level control which would utilise either the same power pack or the same control switch device, or both.

Various other modifications may be made. In particular, an indirectly heated thermistor may be used instead of a self-heating one. Further, the relief outlet part 2 may be dispensed with (the passageway 6 must then lead directly to the interior of the crankcase), and the valve may be used solely to determine whether a minimum pressure difference across the valve is being achieved. A conventional pressure relief valve can then be fitted elsewhere in the pump.

Claims

1. A differential pressure valve, which comprises a valve member movable to allow a bleed flow to pass from an inlet port to an outlet port when the pressure difference across the valve member reaches a first predetermined value, and a sensor for sensing the bleed flow through the valve.

2. A differential pressure valve as claimed in claim 1 , wherein a narrow bore extends from one end to the other of the valve member to provide the bleed flow.

3. A differential pressure valve as claimed in claim 1 or claim 2, wherein there is a narrow clearance between the sides of the valve member and the side wall of the valve chamber.

4. A differential pressure valve as claimed in any one of claims 1 to 3, wherein the valve member is further movable to uncover a pressure relief outlet port when the pressure difference across the valve member reaches a second predetermined value.

5. A differential pressure valve as claimed in any one of claims 1 to 4, wherein the valve member is movable along an elongate valve chamber against the force of biassing means, the inlet port being in one end of the chamber against which the valve member can seat, and the relief outlet port being in the side wall of the chamber spaced apart from the inlet port.

6. A differential pressure valve as claimed in any one of claims 1 to 5, wherein the sensor is a self-heating or indirectly heated thermistor.

7. A differential pressure valve as claimed in any one of claims 1 to 6, wherein the sensor is also capable of sensing wben the temperature of the fluid flow exceeds a predetermined value.

8. A differential pressure valve substantially as hereinbefore described, with reference to, and as shown in, the accompanying drawings.

9. A pump having a differential pressure valve as claimed in any one of claims 1 to 8 associated therewith, the inlet port of the valve being in communication with the discharge side of the pump and the outlet port of the valve being at the same pressure as the suction side of the pump.