Abstract:
An adjustable fail-safe suction stop valve is provided for providing hot gas defrost in refrigeration applications. The dual position valve ( 10 ) utilizes a single solenoid ( 20 ) and a pair of check valves ( 60,62 ) in a configuration that allows the evaporator pressure to be internally equalized through the valve to an adjustable setting over the suction pressure after a defrost cycle has occurred. The intermediate stage allows for this equalization to occur in a controlled manner without the need for an equalization valve.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of the filing date of U.S. Provisional Patent Application Ser. No. 61/263,483, filed Nov. 23, 2009, the disclosure of which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present invention relates, in general, to a solenoid valve such as the type used in hot gas defrost in refrigeration applications, and in particular to an adjustable fail-safe suction stop valve. 
     BACKGROUND 
     Refrigerating systems operating at temperatures below freezing will require defrosting at one time or another. On systems operating below zero degrees Fahrenheit, one common method of defrost is known as hot gas defrost. During the hot gas defrost cycle, the liquid refrigerant (typically ammonia in larger systems) must be pumped out of the evaporator coil, and replaced with warm high pressure ammonia gas. The evaporator is essentially converted to a condenser during this cycle. Upon completion of the defrost cycle, the high pressure gas must be released from the coil back to the compressor suction line. The large volume of compressed gas and the high pressure differences between the evaporator coil and the suction line require the gas be released in a controlled manner to avoid the possibility of vapor propelled liquid in the system. Industry guidelines recommend the use of a small bleed down solenoid valve piped in parallel to the suction stop valve, or valves which open in steps. When incorporating a bleed down solenoid, the smaller valve is opened prior to the suction stop valve for a period of time to allow the pressure in the coil to bleed off gradually. This method is strictly based on time. The use of a bleed down solenoid can still result in problems if the bleed valve does not actually open, or the pressure does not bleed down to a safe level prior to opening the suction stop valve. 
     SUMMARY 
     At least one embodiment of the invention provides a valve assembly comprising: a valve body having spaced inlet and outlet ports separated by an intermediate valve seat in open communication with the inlet and outlet ports; a first piston reciprocal within a bore in the valve body, the piston including a valve member being adapted to sealingly mate with the valve seat; an adapter body secured to an open end of the bore in the valve body; a second piston reciprocal within a bore in the adapter body, the second piston having a portion adapted to extend into the bore in the valve body for contacting the first piston; a solenoid assembly operable to open and close a conduit connecting a source of pressurized gas to the bore in the valve body above the first piston and to the bore in the adapter body above the second piston; and a first check valve in the conduit positioned between the solenoid assembly and the bore in the adapter body above the second piston; a second check valve, the second check valve positioned in a pilot conduit above the second piston; wherein the first check valve opens and the second check valve closes when the solenoid assembly is energized; and wherein the first check valve closes and the second check valve opens when the solenoid assembly is un-energized, the second check valve adapted to close when an inlet pressure to the second check valve falls below the tension of a spring of the second check valve. 
     At least one embodiment of the invention provides a refrigeration valve assembly comprising: a valve body having spaced inlet and outlet ports separated by an intermediate valve seat in open communication with the inlet and outlet ports; a first piston reciprocal within a bore in the valve body, the piston including a valve member being adapted to sealingly mate with the valve seat; a biasing member for normally biasing the valve member away from the valve seat; an adapter body secured to an open end of the bore in the valve body; a second piston reciprocal within a bore in the adapter body, the second piston having a portion adapted to extend into the bore in the valve body for contacting the first piston; a solenoid assembly operable to open and close a conduit from a source of pressurized gas to the bore in the valve body above the first piston and to the bore in the adapter body above the second piston; and a first check valve in the conduit between the solenoid assembly and the bore in the adapter body above the second piston; a pilot conduit connecting the inlet and the bore in the adapter body above the second piston; a second check valve, the second check valve positioned in the pilot conduit above the second piston; wherein the first check valve opens and the second check valve closes when the solenoid assembly is energized; and wherein the first check valve closes and the second check valve opens when the solenoid assembly is un-energized, the second check valve adapted to close when the inlet pressure falls below an activation pressure of the second check valve and the activation pressure of the second check valve is adjustable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of this invention will now be described in further detail with reference to the accompanying drawings, in which: 
         FIG. 1  is a perspective view of an embodiment of the valve in accordance with the present invention; 
         FIG. 2  is a top view of the valve of  FIG. 1 ; 
         FIG. 3  is a cross-sectional view of the valve of  FIGS. 1 and 2  taken along section line A-A in  FIG. 2 ; 
         FIG. 4  is a partial cross-sectional view of the top portion of the valve of  FIGS. 1 and 2  taken along section line B-B in  FIG. 2 ; and 
         FIG. 5  is a detail view of the top portion of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the present invention is shown in  FIGS. 1-5  as an adjustable fail-safe suction stop valve  10  which comprises a stepped opening valve which incorporates an adjustable feature which prevents the valve from opening fully until a desired pressure is reached within the coil. The embodiment shown is an adjustable fail-safe suction stop valve  10  is a normally open, dual piston type stop valve. The valve  10  comprises a valve body  16  having spaced inlet  12  and outlet ports  14  separated by an intermediate valve seat  18  in open communication with the inlet  12  and outlet ports  14 . The valve  10  further comprises a first piston  30  reciprocable within a bore  32  in the valve body  16 , the piston  30  including a valve member  34  being adapted to sealingly mate with the valve seat  18 . A biasing member  36  is positioned for normally biasing the valve member  34  away from the valve seat  18 . An adapter body  72  including a top cover  70  is secured to an open end of the bore  32  in the valve body  16 . A hot gas inlet  80  is shown in the top cover  70 . The valve  10  also includes a second piston  40  reciprocable within a bore  42  in the adapter body  72 , the second piston  40  having a portion  44  adapted to extend into the bore  32  in the valve body  16  for contacting the first piston  30 . The portion  44  of the second piston  40  may be axially adjustable optimize the partial opening of the valve  10  when the second piston  40  is pushed fully downward toward the first piston  30 . The first piston  30  includes a bleed hole  38  through a portion of the first piston  30  providing a fluid pathway connecting the valve body outlet port  14  to the bore  32  in the valve body  16 . 
     A pilot conduit  74  extends from the inlet  12 , the pilot conduit  74  (only partially shown) connectable to passageways to provide high pressure pilot gas to the pistons  30 ,  40  as discussed below. 
     A solenoid assembly  20  is operable to open and close a conduit  22  connecting the high pressure pilot gas from the conduit  74  (connection not shown) to the bore  32  in the valve body  16  above the first piston  30  and to the bore  42  in the adapter body above the second piston. A first check valve  60  is provided in the conduit  22  between the solenoid assembly  20  and the bore  42  in the adapter body  72  above the second piston  40 . 
     A second check valve  62  is positioned along the central axis of the valve  10 . The second check valve  62  is positioned between the pilot gas conduit  74  and the bore  42  in the adapter body  72  above the second piston  40 . The second check valve  62  includes a biasing member  64  (shown as a spring) that has an adjustable tension created by an adjusting stem assembly  90  which allows the axial position of the stem  92  to be set at a particular spring tension. In the embodiment shown a portion of the stem  92  is threaded such that when it is rotated it can increase or decrease the spring tension—thereby controlling the activation pressure of the check valve  62 . This allows the user to adjust the bleed down pressure for where the valve fully opens. In one example, the pressure is set between 5 psi and 40 psi over the suction pressure. The stem  92  protrudes from the valve  10  enabling external adjustment access. The stem assembly  90  may also include a cover  94 . A gage port  78  may be provided with relation to conduit  74  to monitor coil pressure. 
     The valve  10  is closed by energizing the integral solenoid  20 , which opens conduit  22  and supplies high pressure pilot gas to the top of both pistons  30 ,  40  forcing them closed. Pilot gas is fed directly to the main (lower) piston  30  through the pilot porting  22 , while pilot flow to the secondary (top) piston  40  first passes through the first ball check  60 , then directly onto the top piston  40 . 
     Also at this time, the second ball check  62  in the top cover  70  closes and prevents the pilot gas feeding the secondary piston  40  from escaping upstream. The valve  10  stays in this position as long as the solenoid  20  is energized and pilot gas is supplied. As the defrost cycle progresses, pressure will build on the inlet side  12  of the valve  10  to a significantly higher value than that on the outlet side  14  of the valve  10 . 
     At the end of the defrost cycle when the high pressure in the evaporator coil must be equalized, the solenoid coil  20  is de-energized. This terminates pilot gas flow to both pistons  30 ,  40 . At this point the first ball check  60  closes and the second ball check  62  opens, this supplies high pressure gas from the evaporator coil to the secondary piston  40 , holding it shut. The pressure on top of the main piston  30  bleeds out through the bleed hole  38  in the piston  30  creating a low pressure on top of the main piston  30 , allowing it to open. The main piston travels upwards until it hits the bottom  44  of the secondary piston  40  which holds it in a partially open position. The high pressure gas in the evaporator coil can now bleed off through the main valve port  32  in a controlled manner. When the pressure in the evaporator can no longer overcome the adjustable spring tension in the second ball check  62 , pilot flow to the secondary piston  40  is stopped. The gas on top of the secondary piston  40  bleeds out through the internal bleed  76  under the piston  40  and the main piston spring  36  can now push the valve fully open. 
     The key to the function of this valve is in the two ball check assemblies  60 ,  62 . Working in sequence with each other they route pilot pressure from the appropriate location to the secondary piston  40  to hold it shut. They also prevent pilot pressure from being directed to the main piston  30  when the valve is in the equalizing position. 
     The hot gas solenoid valve  10  is a fail-safe valve because if pilot gas to the valve is disrupted in anyway, the valve will not open completely until the pressure in the evaporator has reached a safe level. The valve will go into the equalizing stage and allow the pressure in the evaporator to bleed off slowly. 
     Although the principles, embodiments, and operation of the present invention have been described in detail herein, this is not to be construed as being limited to the particular illustrative forms disclosed. They will thus become apparent to those skilled in the art that various modifications of the embodiments herein can be made without departing from the spirit or scope of the invention.