Abstract:
A vehicle is provided with a controlled internal pressure to preclude intrusion of external atmospheric contaminants. The vehicle has an internally operable egress door. Internal pressure must be maintained low enough to allow occupants to readily open the egress door. A spring-biased poppet valve is used to maintain a requisite very low enclosure-to-atmosphere pressure differential (below 0.2 psid). The valve utilizes a diaphragm and a compression spring as force balancing elements. The compression spring has a spring constant of at least 0.5 pounds per inch.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention is in the field of pressure control and, more particularly, air pressure control of environments in which people may be present. 
         [0002]    In certain circumstances there is a need to maintain a small positive pressure within an enclosure i.e., a pressure greater than atmospheric pressure external to the enclosure. This need may arise in enclosures such as office buildings, manufacturing “clean rooms” or vehicle cabins. A need for accurate pressure control is particularly acute in vehicles that are exposed to widely varying external environmental conditions. 
         [0003]    One example of such a pressure-control need may occur in a military vehicle that may be required to provide nuclear, biological and chemical (NBC) protection for its occupants. In such a vehicle, a positive interior pressure must be maintained at a level higher than external atmospheric pressure. But, the positive pressure must be maintained at a low differential relative to external atmospheric pressure. A pressure control requirement for such a vehicle may be as low as 0.1 pounds per square inch differential (psid) or about 2.8 inches of water. Furthermore ventilation must be provided in such a vehicle. In other words a specified amount of air must continuously flow into and out of the vehicle. Thus an NBC vehicle must be provided with a controlled throughput of air while its internal pressure is maintained at a low positive differential relative to atmospheric pressure. 
         [0004]    The reason for such a low pressure differential requirement relates to structural aspects of the vehicle. In a vehicle with NBC protection, the doors of the vehicle are uniquely configured. A door for an NBC vehicle must be drawn inwardly before it can be opened outwardly. If pressure within the vehicle is too high, it may not be possible to draw the door inwardly for opening. It may be seen that a pressure as small as 0.1 psid will produce a force of about 86 pounds on a typical egress door, e.g., a door with a surface area of about six square feet. It is desirable to provide conditions in which no more than 150 pounds of force may be required to open the door. If interior pressures were allowed to rise to 1 psid then forces on the door would exceed 750 pounds. 
         [0005]    In prior art NBC vehicles, a very low (e.g., 0.2 psid or less) positive pressure differential is controlled and maintained with sensors and numerous interconnected valve actuators and control valves. Complexity of these prior art systems may contribute to high expense and reduced reliability of NBC vehicles. 
         [0006]    Various self-regulating pressure control valves exist in the prior art. Some of these prior art valves are expressly designed to control interior pressure of a vehicle such as aircraft cabin pressure. One example of such a prior art valve is a balanced poppet valve. Balanced poppet valves are used extensively in aircraft. But pressure setpoint requirements for aircraft are not as low as the requirements for an NBC vehicle. Typically, control of aircraft cabin pressure is considered adequate with setpoints down to about 0.9 psid. Prior art balanced poppet valves perform successfully at that level of pressure control. 
         [0007]    Prior-art balanced poppet valves have provided simple and cost-effective pressure control in aircraft for decades. But, these prior art valves are not capable of providing required very low pressure control in NBC vehicles. Typically, these valves are constructed so that a desired pressure may be indirectly maintained through an ancillary metering valve. The metering valve operates with a control spring having a spring constant no lower than about 0.5 pounds per inch (lb/in) of compressive deflection. Control springs with spring constants lower than 0.5 lbs/in may not be suitable for use in poppet valves because they may not provide the accuracy required. Consequently a spring constant of 0.5 lbs/in is considered a lower limit for control springs used in poppet valves. This spring-constant lower limit translates into a lower limit of about 0.9 psid of pressure control capability for prior-art poppet valves with an ancillary metering valve. 
         [0008]    As can be seen, there is a need to provide a simple pressure control system that may operate in a range of pressures below about 0.2 psid. Additionally, there is a need to provide a self-regulating valve that will control pressure at this low range. 
       SUMMARY OF THE INVENTION 
       [0009]    In one aspect of the present invention, an apparatus for maintaining a desired pressure of air flowing through an enclosure comprises a valve with an outlet in communication with an atmosphere outside of the enclosure and an inlet in communication with air inside the enclosure and a pressure-control member in the valve. The pressure-control member is directly displaceable responsively to an enclosure-to-atmospheric air pressure differential. The pressure-control member is spring biased to modulate air flow through the valve to maintain the enclosure-to-atmospheric air pressure differential within a predetermined range. 
         [0010]    In another aspect of the present invention, a vehicle with a ventilated enclosure for transporting occupants in a hazardous atmospheric environment comprises a self-regulating spring-biased valve having a modulating pressure-control member. The valve has an outlet in communication with an atmosphere surrounding the vehicle. The pressure-control member is movable with force produced by an enclosure-to-atmosphere pressure differential. 
         [0011]    In still another aspect of the present invention, a method for maintaining a desired air pressure within a ventilated enclosure comprises the steps of forcing air into the enclosure and releasing air from the enclosure through a self-regulating spring-biased valve by directly displacing a pressure-control member with force produced by an enclosure-to-atmosphere pressure differential. 
         [0012]    These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is block diagram view of an enclosure with a pressure control system in accordance with the invention; 
           [0014]      FIG. 2  is a cross-sectional view of a pressure regulating valve in accordance with the invention; and 
           [0015]      FIG. 3  is a flow chart of a method of controlling pressure in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
         [0017]    Broadly, the present invention may be useful for controlling air pressure in stationary or mobile enclosures, such as vehicles, clean rooms or buildings. More particularly, the present invention may provide for maintaining and controlling very low pressure in such enclosures. The present invention may be particularly useful in vehicles such as military vehicles. 
         [0018]    In contrast to prior-art pressure control systems, among other things, the present invention may maintain a tightly controlled very low positive pressure in a cabin of a vehicle with a self-regulating pneumatic control valve. The present invention may, instead of employing complex combinations of prior-art electronic sensors, valve actuators and valves, utilize a balanced poppet valve to maintain pressure in a control range of about 0.2 psid or less. 
         [0019]    Referring now to  FIG. 1 , a block diagram portrays a typical NBC vehicle, designated generally by the numeral  100 . The vehicle  100  may comprise a vehicle enclosure or cabin  102 , an air inflow device  104  and an air outflow valve  106 . The vehicle  100  may be an NBC vehicle with an inwardly operable egress door  108  that may be readily opened by occupants of the vehicle  100 . The NBC vehicle  100  may be provided with a positive internal pressure to assure that external atmospheric contaminants do not enter the vehicle. However, the internal pressure may not be allowed to become great enough to interfere with operability of the egress door  108 . These conditions may require maintenance of a very low positive pressure differential between cabin pressure Pc and external atmospheric pressure Pa. The required pressure differential may be as low as 0.2 psid or less. 
         [0020]    In operation, the air inflow device  104  may draw filtered atmospheric air into the cabin  102  and thereby ventilate and pressurize the cabin or enclosure  102 . The air inflow device  104  may comprise an air de-contamination system (not shown) and a pressurizing device such as a fan or compressor (not shown). The air inflow device  104  may operate continuously to deliver de-contaminated air into the cabin  102 . A desired pressure Pc may be maintained in the cabin  102  because the air outflow valve  106  may permit air to escape from the cabin  102  whenever the desired pressure Pc is reached. 
         [0021]    Referring now to  FIG. 2 , there is shown a balanced poppet valve which may perform the role of the air outflow valve  106  of  FIG. 1 . The balanced poppet valve of  FIG. 2  is designated generally by the numeral  106 . The valve  106  may comprise an upper body  112 , a lower body  114 , a pressure-control member or poppet  116 , a spring  118  and a diaphragm  122 . 
         [0022]    In operation the valve  106  may be positioned within the vehicle  100  of  FIG. 1  so that an outlet port  114   a  may be exposed to external atmospheric pressure Pc and inlet ports  114   b  may be exposed to internal cabin pressure Pc. When cabin air pressure exceeds external atmospheric pressure by a predetermined differential (Pc-Pa), the poppet  116  may move axially to allow air to escape from the cabin  102 . 
         [0023]    As air flows into the cabin  102  of  FIG. 1  the pressure differential (Pc-Pa) may increase. The poppet  116  may move axially to allow some of the cabin air to escape to the atmosphere. As an amount of inflowing air increases, the pressure differential (Pc-Pa) may increase. In that case the poppet  116  may increase its axial movement. Such an increase in axial movement of the poppet  116  may result in increased air flow into the inlet ports  114   b  and out through the outlet port  114   a . A balanced flow may develop through the cabin  102  as the valve  106  releases air from the cabin  102  at a sufficient rate to maintain a desired enclosure-to atmosphere pressure differential within the cabin  102 . A pressure differential of about 0.1 psid to about 0.2 psid may be maintained. The valve  106  may directly control the pressure differential without use of an ancillary metering valve. 
         [0024]    It may be noted that the diaphragm and attached (hereinafter called diaphragm)  122  may comprise a rigid poppet plate  122 - 1  and a flexible annular member  122 - 2  in communication with the cabin  102  on one side and the atmosphere on the other side. The diaphragm  122  may be attached to the poppet  116 . In a presence of a positive (Pc-Pa), the diaphragm  112  may produce an axial displacing force on the poppet  116 . In the inventive valve  106 , the diaphragm  122  may be large enough to produce a significant axial force on the spring  118 . Axial force produced by the diaphragm  122  may be counteracted by axial forces produced by the spring  118 . This arrangement may uniquely allow for control of very low pressures while employing a spring with that performs with proper repeatability. 
         [0025]    This may be better understood by considering an exemplary dimensional relationship of various elements of the valve  106 . In a typical one of the vehicle cabins  102 , a limited volume of space may be available for the valve  106 . As an example, a space envelope for the valve  106  may be specified at a size of about 6 inches in diameter and about 2 inches deep or about 60 cubic inches. Thus, an exemplary one of the valves  106  may have a maximum diameter and depth that fits into the specified space envelope. Within that limited space envelope, the valve  106  may be required to have air passages large enough to release air from the cabin  102  at a specified rate and at a specified low (Pc-Pa). 
         [0026]    In order for the spring  118  to have a spring constant of at least about 0.5 lbs/in the poppet  116  may require displacement against a spring force of at least two pounds. Displacement of the poppet  116  may be provided with force developed by the diaphragm  122 . At a low pressure of about 0.1 psid, a two pound displacement force may require that the diaphragm have a surface area of about twenty square inches. If the diaphragm  122  were any smaller, the poppet  116  may not be held in a balanced position with a spring having a spring constant of 0.5 lbs/in. In other words, if the diaphragm  122  were smaller, the spring  118  might need to be constructed with a spring constant less than 0.5 lbs/in. As described hereinbefore, a spring constant of 0.5 lbs/in may be a lower limit for poppet valve compression springs. Compression springs with spring constants lower than 0.5 lb/in may not provide reliable and repeatable deflection performance. 
         [0027]    The inventive valve  105  may be constructed so that the surface area of the diaphragm  122  is large enough to produce a counteracting force on the spring  118  which force is large enough to permit construction of the spring  118  with a spring constant of at least about. 0.5 lbs/in. This may be accomplished by allowing the cabin pressure to create a force on one side of the diaphragm  122  which opposes a combined atmospheric pressure and spring force on the other side of the diaphragm  122 . When such a configuration is provided, the surface area of the diaphragm  122  may be made relatively large while permitting a presence of a relatively large poppet  116  in the valve  106  in spite of the valve  106  having limited size. In the exemplary valve  106  discussed hereinabove, the valve body  114  may have an exterior diameter of about 6 inches. The diaphragm  122  may have a surface area of about 20 square inches and the poppet  116  may have a diameter of about 5 inches. Thus the poppet  116  may have a diameter that is more than 80% of the diameter of the valve body  114 . Such an arrangement, with a relatively large poppet  116  provides an opportunity for air flow of up to about 240 cubic feet per minute through the valve  106  at a pressure of about 0.1 psid. 
         [0028]    Consequently, one of the NBC vehicles  100  may be provided with a generous volume of ventilation air for its occupants even though the valve  106  may occupy a space envelope no larger than about 60 cubic inches in the vehicle  100 . Furthermore, the inventive valve  106  may control a requisite internal pressure in the vehicle  100  without any sensing devices or control elements that may be external to the valve  106  itself. 
         [0029]    In one embodiment of the present invention, a method may be provided for maintaining a desired air pressure within a ventilated enclosure (e.g. the vehicle cabin  100 ). In that regard the method may be understood by referring to  FIG. 3 . In  FIG. 3 , a flow chart portrays various aspects of an inventive method  300 . In a step  302 , air may be continuously introduced into the enclosure (e.g. by the inflow device  104 ). In a step  304 , air may be introduced into an inlet of a valve (e.g. the inlet  114   b  of the valve  106 ). In a step  306 , a pressure-control member (e.g., the poppet  116 ) may move responsively to an enclosure-to atmosphere pressure differential (e.g., movement of the poppet  116  is responsive to [Pc-Pa]). In a step  310 , a spring force may be maintained on the pressure-control member to limit its movement (e.g. the spring  118  may produce force against the poppet  116 ). In a step  312 , enclosure-to-atmospheric pressure differential may be maintained with modulating axial movement of the pressure-control member. 
         [0030]    It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.