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[amolbhvsr]

I am working on refrigeration system relief design. We have two queries as mentioned below:

 

1)            The required relief rate calculation is based on the equation presented in the ASHRAE-15 Safety Standard for Refrigeration Systems:

C = f x L x D   per Section 9.7.5, ASHRAE Standard 15-2001

Where,

C = required relief rate in equivalent units of air (lb/min)

f = refrigerant factor, increased by a factor of 2.5 for cases where the equipment is within 20 feet of a combustible fluid source

L = length of vessel (ft)

D = diameter of vessel (ft)

 

While calculating the relief load for Refrigeration evaporator/condenser, if the tube side contains the refrigerant, then what should be the basis of L & D? Does the refrigerant factor “f” considers that heat will be transferred from the shell side to the tube side? Now per ASHRAE-15, the fire basis is heat generated from fire radiating on a projected area i.e. equipment is not fully engulfed in fire. So how to calculate the relief load for such cases? Can we consider the total exchanger length (L) and the shell OD (D) for calculating the fire relief load?

 

2)            Also, the fire required rate is in lb of air/min. Can we convert the lb of air/min to equivalent            refrigerant and then size the relief valve?

ASHRAE provides an equation for calculating the allowable equivalent length at relief valve outlet. This equation is based on the air capacity of valve, which states that the allowable backpressure at conventional relief valve is 15%. So is this applicable when the outlet pressure drop is calculated in terms of equivalent refrigerant flowing out of valve? We came across cases where there is a huge difference in outlet pressure drop. With air the outlet pressure drop is 15%, while with refrigerant the value is 28%. This is due to the difference in specific heat ratio considered. ASHRAE-15 recommends isothermal compressible flow basis at relief valve outlet to calculate the pressure drop.

So the question is in practical, the refrigerant will be relieved through the valve, hence the pressure drop calculated should be based on refrigerant. So what forms the basis of allowable equivalent length calculation equation?