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Refrigerant Leak Rate Relationships

The requirement to monitor machine rooms for refrigerant leaks has brought up questions relating to leak rate of a refrigerant and readings of the monitoring system.  Industry practice has been to think of refrigerant leaks in terms of pounds of refrigerant per unit time, such as pounds per hour, or more often, ounces per year.  This is a logical way of looking at refrigerant leaks.

All systems look at the amount of refrigerant present in air in parts per million (PPM).  That is; the volume of refrigerant molecules present as compared to air molecules present.  A relationship between a leak rate in weight per unit time and PPM reading can be ascertained.  There are a number of things that need to be accounted for.  They are:

1-- Room volume

2-- The relationship between refrigerant amounts in weight compared to refrigerant volume at the temperature and pressure of the room.

3-- The amount of time the refrigerant has been leaking.

4-- The rate at which fresh air enters the room and stale air is exhausted.

5-- The location of the refrigerant monitor inlet relative to the leak, the air patterns of the room, and the rate at which the leaking refrigerant expands to fill the room.

For a given situation, items 1 through 4 are either known, can be found, or can be estimated.  Item 5 is unknown.  In place of item 5, one can assume the leaking refrigerant expands immediately to fill the room.  This is a reasonable assumption based on having located the monitor as per the monitor manufacturer’s recommendations.  This should result in the monitor seeing a higher concentration sooner than the idealized formula.

Monitored machine rooms can be classified into one of two categories:  One will be called a “sealed room”.  A sealed room is one that has no induced air turnover, no exhaust or intake fans to promote fresh air circulation.  The other is a room with a known turnover of air through the use of a fan.

In order to convert between a leak rate in cubic feet per hour to a leak rate in pounds per hour, the molecular weight of the refrigerant needs to be known. 

At normal atmospheric pressure and temperature for the most common refrigerants, these conversation factors are:

Conversion factors:

R-22    =  0.22 lbs./cu. ft. or 4.46 cu. ft./lb.

R-12    =  0.31 lbs./cu. ft. or 3.18 cu. ft./lb.

R-11    =  0.36 lbs./cu. ft. or 2.80 cu. ft./lb.

R-502  =  0.29 lbs./cu. ft. or 3.45 cu. ft./lb.

R-123  =  0.41 lbs./cu. ft. or 2.41 cu. ft. lb.

In the examples, the definitions for the formulas used are:

PPM    =    Monitor PPM reading or trip point PPM setting

LR       =    Leak rate in cubit feet per hour

FA        =    Fresh air into a room in cubic feet per hour

VOL     =    Volume of a room in cubic feet

T          =    Time in hours (8760 hours in a year)

R         =    Amount of refrigerant in a room in cubic feet

LRMIN =    Minimum leak rate that will reach a given PPM

Some formulas for the “sealed room”:

 

        As an example, if a sealed room is 40' long by 30' wide and 10' high, and we want to find out how much refrigerant 22 would cause a 25-PPM reading:

Find the volume of the room

 

Another example:

In the same room, we know we are leaking 300 lbs. per year of R-22.  How long will it take to get a 25-PPM reading?

First, find how many cubic feet an hour is leaking.

 

The formulas change for a room that has exhaust and intake air fans changing the air.  One of the formulas is:

LRMIN = PPM x FA x 10-6 

(There are other formulas to calculate T and PPM for rooms with air changes, but they involve natural logarithms of numbers.  Unless one has, and understands, a scientific calculator, they are very hard to solve.)

Assume we are dealing with the same room, but fans have been added to give us fresh air at 100 CFM, which is 6000 Ft3/Hr, or one air change every two hours.  What is the minimum leak rate of refrigerant that will reach 25-PPM?

Solving the formula, we get:

LRMIN = 25 x 6000 x 10-6

LRMIN = .15 cu. ft./hr

If the refrigerant is R-22, .22 lbs./cu. ft., the leak rate is .033 lbs. hr. or 289 lbs./year

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