UFC 3-420-01
25 October 2004
following:
Qs = 2.5 x [(11050)/(14050)]
Qs = 1.667 gpm of hot water from storage tank
Qc = QfQs = 2.5 gpm1.667 gpm = 0.833 gpm of cold water from supply
The mixing equation may be used to evaluate the effect of variation of Tc
temperatures on Qs. At Ts = 140 F, for Tc = 40 F, Qs = 1.75 gpm; and for Tc
= 80 F, Qs = 1.25 gpm.
The mixing equation may also be used to evaluate the required size of SWH
storage tanks as Ts varies. Higher storage temperatures allow smaller tank
sizes to deliver equal water to fixtures.
506.1.4 Revisions to ASHRAE Chapter 48, Table 9, Row 9, Showers.
Table 9, Row 9, Showers, indicates 225 gallons per hour (gph) per fixture for
hot water flow in Gymnasiums, Industrial Plants, Schools, and YMCA's.
Assume this represents continuous flow of shower fixture, what is the design
fixture flow rate? Since 225 gph/60 min. per hour = 3.75 gpm of hot water
flow = Qs, calculate the fixture flow Qf from the mixing equation, using Tf =
110 degrees F, Ts = 140 degrees F, and Tc = 50 degrees F. Observe that Qf
= Qs + Qc, therefore Qc = Qf Qs. Substituting:
Qf x Tf = (Qs x Ts) + (Qc x Tc)
Qf x Tf = (Qs x Ts) + ((QfQs) x Tc)
Qf x Tf = (Qs x Ts) + (Qf x Tc) (Qs x Tc)
(Qf x Tf) (Qf x Tc) = (Qs x Ts) (Qs x Tc)
Qf x (TfTc) = Qs x (TsTc)
Qf = Qs x [(TsTc)/(TfTc)]
Qf = 3.75 x [(14050)/(110-50)]
Qf = 5.625 gpm
Therefore, the proper correction factor (CF) for Chapter 48, Table 9, Row 9 is
equal to 2.5 gpm per fixture (per PL 102-486) divided by 5.625 gpm
(calculated above), which results in CF = 0.444 for Tc of 50 degrees F. This
may also be calculated as Qs = 1.667 divided by Qs = 3.75 equals CF =
0.444.
Thus, the gymnasium shower at 225 gph x CF corrects to 225 gph x 0.444 =
100 gph for Tc of 50 degrees F.
506.1.5 Domestic Hot Water Recirculation Pumps. For Navy, Army and
Air Force projects, see Appendix E of UFC 3-420-01, Design: Plumbing
Systems.
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