GB2222778A

GB2222778A - Positive pressure filter respirator

Info

Publication number: GB2222778A
Application number: GB8821998A
Authority: GB
Inventors: Keith Simpson
Original assignee: Sabre Safety Ltd
Current assignee: Sabre Safety Ltd
Priority date: 1988-09-19
Filing date: 1988-09-19
Publication date: 1990-03-21
Anticipated expiration: 2008-09-19
Also published as: GB8821998D0; GB2222778B

Abstract

The flow of air into a positive pressure filter respirator during exhalation is reduced without having to provide an inhalation valve by using an exhale valve which is a flap valve having a high cracking resistance but a low resistance to flow. The exhale valve has a pressure/flow characteristic (C, Fig. 1) in which the pressure resistance increases with flow from a cracking pressure greater than 10 mm water gauge at 30 litres per minute flow until at a flow rate in the region of 70 - 80 litres per minute, the pressure resistance across the valve falls such that the pressure across the exhale valve at a rate of flow of 85 litres per minute through the valve is less than 12.5 mm water gauge. This effect is achieved by using an exhale valve which is a flap valve having a flexible flap (2) connected to a circular stem (3) by a resilient fold (1) which permits the flexible flap (2) to move axially relative to the circular stem (3) as the rate of flow through the valve increases past 70 litres per minute. <IMAGE>

Description

POSITIVE PRESSURE FILTER RESPIRATORS This invention relates to positive pressure filter respirators.

In positive pressure filter respirators a positive pressure greater than the ambient pressure is maintained within a face mask surrounding the respiratory passages of a wearer by means of an air mover such as a blower or a fan which delivers air through a filter to the interior of the face mask. The air mover, which may be a battery-driven electrical device, removes from the wearer the need to overcome the resistance of the filter and so reduces the breathing effort required.

The use of positive pressure has the particular advantage that, if there is any leakage, the leakage will all be outward from the face mask thus providing a high degree of protection against the ingress of noxious fumes into the gas which the wearer of the respirator is breathing.

In order to avoid undue exertion by the wearer during his breathing cycle it is a requirement for positive pressure filter respirators that their resistance to exhalation at a flow rate of 85 litres per minute shall be no greater than 12.5mms water gauge.

In consequence of this requirement there have been used, in positive pressure filter respirators, exhalation valves which have a very low cracking pressure.

The pressure/flow characteristic of a conventional valve which has a resistance of less than 12.5mms water gauge at a flow rate of 85 litres per minute through the valve is such that the pressure resistance of the valve at a flow rate of 30 litres per minute is substantially less than Smms water gauge.

With such a conventional exhalation valve used in a positive pressure filter respirator it has not been possible to generate a pressure within the respirator sufficient to cause the air mover to reduce the flow of air into the respirator during exhalation unless an inhalation valve is also used.

In accordance with the present invention there is provided a positive pressure filter respirator comprising a face mask for engaging the head of a wearer to surround his respiratory passages, an air inlet to the mask, a filter mounted to the mask to filter air passing through the air inlet, air moving means for moving air through the filter into the mask to establish a pressure above ambient pressure within the mask, and an exhale valve mounted within the mask, the exhale valve having a characteristic such that the pressure across the exhale valve at a rate of flow of 30 litres per minute through the exhale valve is greater than 8mms water gauge and preferably greater than 10mms water gauge, and the pressure across the exhale valve at a rate of flow of 85 litres per minute through the exhale valve is less than 12.5mms water gauge.

In the embodiment of the invention which will be described there is used an exhale valve which has such a characteristic and which is a flap valve having a flexible flap connected to a central stem by a resilient fold which permits the flexible flap to move axially relative to the central stem.

The present invention will be further understood from the following detailed description which is made with reference to the accompanying drawings, in which: Figure 1 shows the pressure/flow characteristics of two known conventional valves and of a valve used in a preferred embodiment of a positive pressure filter respirator in accordance with the present invention, and Figure 2 consists of three cross-sectional views through a preferred valve having the pressure/flow characteristic utilised in the present invention, the views showing the construction of the valve and the movements of the valve from the rest position in response to an increasing rate of flow through the valve.

For the general construction of a positive pressure filter respirator reference may be made to our co-pending UK Patent Application No. 8806713.

Figure 1 of the accompanying drawings is a graph showing at A, B and C the pressure/flow characteristics of three flap valves. There is also denoted in Figure 1 at X the pressure resistance of 12.5mms water gauge at a flow rate of 85 litres per minute through the valve.

It is evident therefore that, for a valve to meet the requirements of an exhalation valve for use in a positive pressure filter respirator, the pressure/flow characteristic of the valve must pass below point X in Figure 1.

Of the two conventional flap valves whose characteristics are illustrated at A and B in Figure 1, valve A has a substantial cracking pressure (a pressure resistance of more than 12mms water gauge at a flow rate of 30 litres per minute) but its characteristic passes well above point X, so that valve A cannot be used in positive pressure filter respirators.

The characteristic of valve B, on the other hand passes below point X, so that valve B is suitable for use in positive pressure filter respirators. However the cracking pressure of valve B is very low (less than 3mms water gauge at a flow rate of 30 litres per minute).

Characteristic C is the pressure/flow characteristic of the valve used in accordance with the present invention and which is illustrated in Figure 2. Valve C has a pressure resistance in excess of 10mms water gauge at a rate of flow through the valve of 30 litres per minute, the pressure increasing as the rate of flow rises to about 70 litres per minute. There is then a fall in pressure resistance across the valve as the rate of flow increases from 70 to 80 litres per minute, thus ensuring that the pressure resistance of the valve is less than 12.Smms water gauge at 85 litres per minute.

The pressure resistance across the valve then continues its gradual rise with further increases in the flow rate of gas through the valve.

Referring to Figure 2a the valve of characteristic C is a rubber flap valve made with an annular fold 1 closely adjacent to the junction between the flap or mushroom 2 and a central axial valve stem 3. In the closed position of the valve shown in Figure 2a, the flap or mushroom 2 is in contact with a surface 4 surrounding an orifice 5 through which the stem 3 passes.

As valve C is subjected to increased gas pressure, so the outside circumference of the flap or mushroom 2 bends gently away from the surface 4 as shown in Figure 2b, permitting gas flow through the valve. However, when the rate of flow through the valve has increased to about 70 litres per minute, the fold 1 between the flap or mushroom 2 and the stem 3 is extended so that the whole flap or mushroom 2 moves axially relative to the stem 3, thus increasing the space through which gas may flow as shown in Figure 2c. This action of the valve causes the reduction in pressure resistance across the valve illustrated in characteristic C of Figure 1 between the flow rates of 70 and 80 litres per minute which results in characteristic C meeting the requirement for an exhalation valve in positive pressure filter respirators.

By making the valve illustrated in Figure 2 of rubbers of different stiffnesses, a family of valves having characteristics of shape similar to C may be obtained.

The use of an exhale valve having a high cracking resistance but a low resistance to flow, as described hereinbefore, in positive pressure filter respirators has the advantage of controlling the flow through the filter during the exhale part of the breathing cycle, thus increasing the life of the filter and of the battery which drives the fan motor.

Claims

CLAIMS:

1. A positive pressure filter respirator comprising a face mask for engaging the head of a wearer to surround his respiratory passages, an air inlet to the mask, a filter mounted to the mask to filter air passing through the air inlet, air moving means for moving air through the filter into the mask to establish a pressure above ambient pressure within the mask, and an exhale valve mounted within the mask, the exhale valve having a characteristic such that the pressure across the exhale valve at a rate of flow of 30 litres per minute through the exhale valve is greater than 8mms water gauge and the pressure across the exhale valve at a rate of flow of 85 litres per minute through the exhale valve is less than 12.5mms water gauge.

2. A positive pressure filter respirator 'according to Claim 1 wherein the exhale valve is a flap valve having a flexible flap connected to a central stem by a resilient fold which permits the flexible flap to move axially relative to the central stem.

3. A positive pressure filter respirator according to Claim 1 or Claim 2 wherein the exhale valve has a characteristic such that the pressure across the exhale valve is greater than 10mms water gauge at a rate of flow of 30 litres per minute.

4. A positive pressure filter respirator substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.