MSD "CAPACITY" CALCULATION
MSD capacity is defined in terms of the amount of gas that can be treated prior to breakthrough of water at 1 ppm. For convenience, the definition includes the number of hours (required by the customer) to breakthrough, gas feed rate and gas moisture content.
e.g. 100 hrs @ 100 SCFH @ 0.1% water in feed = 1000 SCFH-%-hrs
100 hrs @ 10 SCFH @ 1% water in feed = 1000 SCFH-%-hrs
MSD dryers can treat gas flows at rates equal to or smaller than the "capacity" indicated by the model number. An MSD-100, for instance, can treat up to 100 SCFH of gas. If the flow rate is larger than 100 SCFH, a larger dryer should be specified, even though the MSD-100 might have adequate capacity on the basis of breakthrough time.
EXAMPLES OF USE OF GRAPH
(A) The flow rate of the feed gas is 100 SCFH. The dew point of the feed gas is -20F when tested at 1 atm. The minimum allowable running time between regenerations is 100 hours. Find the smallest appropriate dryer.
Draw a vertical line to the curve from and a horizontal line to the right axis from the curve. Read approximately 0.04% H2O by volume.
0.04% x 100 SCFH x 100 hrs = 400 SCFH-%-hrs
The smallest unit with sufficient capacity is the MSD-500.
(B) A feed gas is at 5 atm. The dew point of the product gas from an MSD is -100F at 5 atm. Find the dew point of the dried product gas when expanded to 1 atm.
Draw a vertical line from -100F and a horizontal line to the left axis from the curve. Read approximately 0.001 mmHg. Divide 0.001 by 5. The reduced vapor pressure is approximately 0.0002 mmHg. Draw a horizontal line from 0.0002 mmHg to the curve and a vertical line to the temperature scale. Read a dew point of -120F for the expanded product stream.