SenTech IR-SNIF-MCD Series - Page 44

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Two examples have been developed to demonstrate the conversion. Case I is for a
sealed room, with no air turnover. Case II is for a room with a known air turnover.
Case I: Sealed room 40 ft x 30 ft x 10 ft
high
1. How much R-22 (in lbs) is necessary to
cause a measurement of 25 ppm?
Given 25 ppm R-22 in the sealed room:
Volume of R-22 = parts per million / million x room
volume
R(cuft) = [25(ppm)/1,000,000] x [40 x 30 x 10](cuft)
R(cuft) = .3 cuft of R-22 in the sealed room
Given .3 cuft R-22, calculate weight in lbs:
Weight = volume(cuft) x density(lb/cuft)
Weight = R(cuft) x RD22(lb/cuft)
Weight = .3(cuft) x .22(lb/cuft)
Weight = .066 lbs
2. If the leak rate is 300 lb/yr, how long will
it take to reach 25 ppm?
Given the same refrigerant in the same
room, 25 ppm weighs .066 lbs:
Time(hr) = weight of R-22(lb) / [weight(lb)/time(yr)] x
[time(hr)/time(yr)]
Time(hr) = .066(lb) / [300(lb)/1(yr)] x [8760(hr)/1(yr)]
Time(hr) = 1.93 hrs
Case II: 40 ft x 30 ft x 10 ft high room with
fresh air makeup of 100 cfm
1. How much R-22 (in lbs/yr) is necessary
to cause a measurement of 25 ppm?
Given air turnover of 100cfm, calculate the
fresh air volume in cuft/hr:
Air flow (cuft/hr) = air flow (cfm) x [60(min)/1(hr)]
Air flow(cuft/hr) = 100cfm x 60 min/hr = 6000 cuft/hr
Given 6000 cuft/hr, calculate the leak rate
that is required to maintain a measurement
of 25 ppm:
Leak rate min (cuft/hr) = parts per million/million x fresh
volume(cuft/hr)
Leak rate min (cuft/hr) = [25(ppm)/1.000.000] x 6000(cuft/hr)
Leak rate min(cuft/hr) = .15 cuft/hr
Given .15 cuft/hr, calculate the leak rate in
lb/yr needed to reach 25 ppm:
Leak rate(lb/yr) = leak rate(cuft/hr) x density(lb/cuft) x [8760(hr) /
1(yr)]
Leak rate(lb/yr) = .15(cuft/hr) x .22(lb/cuft) x 8760(hr/yr)
Leak rate(lb/yr) = 289 lb/yr, therefore the leak rate must be
GREATER THAN 289 lb/yr.
2. If the leak rate is 500 lb/yr, how long will
it take to reach 25 ppm?
Calculate LR in cuft/hr:
LR(cuft/hr) = [leak rate(lb/yr) / density(lb/cuft)] / 8760(hr/yr)
LR(cuft/yr) = 500(lb/yr) / .22(lb/cuft) / 8760(hr/yr)
LR(cuft/yr) = .26 cuft/hr
Calculate time in hr:
Time(hr) = [Room volume(cuft)/Air flow(cuft/hr)] x ln[LR(cuft/hr) /
[LR(cuft/hr) – Leak rate min(cuft/hr)]]
Time(hr) = [12000(cuft)/6000(cuft/hr)] x ln[.26(cuft/hr)/[.26(cuft/hr)-
.15(cuft/hr)]
Time(hr) = 2(hr) x ln[2.3636] Time(hr) = 1.72 hr