MIL-HDBK-1191
(e) Use care to properly select the operating head of the
recirculation pump(s), taking into consideration system head
requirements, including the pressure drop of thermostatic mixing valves.
(f) Consider instantaneous hot water generators where
feasible, and when on-site storage is not required by other criteria, to
avoid stagnant storage conditions.
(g) Water storage of holding tanks, both cold and hot, shall
be designed to have continuous flow with maximum practical velocity.
They shall not be piped as branched components with discharge line
shutoff valves closed. Storage tank capacity shall be adequate for the
application, but not oversized. The capacity sizing shall factor in the
current healthcare procedures such as increased outpatient medical
functions reducing the load profiles and hot water recovery demand.
9.2.6
Domestic Hot Water Systems.
Hot Water Demand. The demand load can be calculated using
9.2.6.1
several different methods, examples of which are contained in ASHRAE and
American Society of Plumbing Engineers (ASPE) handbooks (references 9h or
9i). The preferred method for calculating hot water demand for an MTF is
the per-fixture method. This method provides a means to account for
various uses within each facility, and permits the tabulation of the hot
water load using a specified demand for each fixture. Summing individual
fixture demands, the total demand is utilized to size the heater recovery
rate and storage size. Table 9-3 provides a list of representative
fixtures taken from MIL-STD-1691 (reference 9j) and their demands based
on data provided in ASHRAE and ASPE handbooks (references 9h and 9i). For
any fixture not shown, the designer shall determine the demand rate based
upon experience, standard practice, and available Using Agency input. An
example of system sizing is given below:
Example 1: Determine the heater and storage tank size for a medical
clinic with the following fixture schedule:
12
Clinic Sinks
X
15.0
L/h
( 4
gph)
=
180.0
L/h
( 48
gph)
1
Plaster Sink
X
38.0
L/h
( 10
gph)
=
38.0
L/h
( 10
gph)
8
Public Lavatories
X
23.0
L/h
( 6
gph)
=
184.0
L/h
( 48
gph)
12
Clinic Lavatories
X
30.0
L/h
( 8
gph)
=
360.0
L/h
( 96
gph)
2
Mop Service Basins
X
76.0
L/h
( 20
gph)
=
152.0
L/h
( 40
gph)
1
Arm Whirlpool Bath
X
130.0
L/h
( 35
gph)
=
130.0
L/h
( 35
gph)
1
Leg Whirlpool Bath
X
380.0
L/h
(100
gph)
=
380.0
L/h
(100
gph)
1
Foot Whirlpool Bath
X
57.0
L/h
( 15
gph)
=
57.0
L/h
( 15
gph)
1
X-ray Film Processor
X
57.0
L/h
( 15
gph)
=
57.0
L/h
( 15
gph)
Possible Maximum Demand
1538.0 L/h
(407 gph)
Using the Demand Factor of 0.40 yields
Probable Maximum Demand
=
Possible Maximum Demand X Demand Factor
Recovery Rate
=
1543.6 L/h X 0.40 (407 gph X 0.40)
Heater or Coil Capacity
=
617.4 L/h (162.8 gph)
The Storage Capacity Factor of 1.00 is then applied to this recovery rate
or heater capacity to determine the
Storage Tank Capacity
=
Heater Capacity X Storage Capacity Factor
=
617.4 X 1.00 (162.8 X 1.00)
=
617.4 L (162.8 gal)
9-8
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