DTH Electric Boilers USE & CARE MANUAL
(Revision June 2014)
, Page
9.
Table 6: Temperature rise vs flow rate (GPM)
BWTD
Model
KW
10
o
F
20
o
F
30
o
F
40
o
F
DTH 42
42
28,8
14,4
9,6
7,2
DTH 45
45
30,8
15,4
10,3
7,7
DTH 48
48
32,9
16,4
11,0
8,7
DTH 54
54
37,0
18,5
12,3
9,2
DTH 60
60
41,1
20,6
13,7
10,3
DTH 66
66
45,2
22,6
15,1
11,3
DTH 72
72
49,3
24,7
16,4
12,3
DTH 78
78
53,4
26,7
17,8
13,4
DTH 84
84
57,6
28,8
19,2
14,3
DTH 90
90
61,7
30,8
20,6
15,4
DTH 96
96
65,8
32,9
21,9
16,4
DTH 99
99
67,8
33,9
22,6
17,0
DTH 102
102
69,6
34,8
23,2
17,4
DTH 108
108
73,7
36,8
24,6
18,4
DTH 120
120
82,2
41,1
27,4
16,1
DTH 132
33
90,5
45,2
30,2
22,6
DTH 144
36
98,7
49,3
32,9
24,7
3.4.9.3 Pipe sizing criteria
Proper selection of pipe size is important to
efficient system operation. A large pipe size
results in lower friction losses and may allow the
selection of smaller, more economical pump.
The increased pipe size, however, costs more
initially and must be balanced against the cost
savings realized be smaller pump. Likewise,
small pipe costs less initially but must be
balanced against the increased operating cost of
pumping water through a system with high
friction losses. An economical balance should
be reached between pump size, operating costs,
and pipe diameter.
The ASHRAE fundamentals handbook states
the general range of pipe friction loss used for
the design of hydraulic systems and upper limits
of water velocity in piping.
A variety of upper limits of water velocity and/or
pressure drop in piping and piping systems are
used. One recommendation places a velocity
limit of 4 feet per second for 2 inch pipe and
smaller, and a pressure drop limit of 4 feet of
water per hundred feet for piping over 2 inches.
These limitations are imposed either to control
the levels of pipe and valve noise, erosion and
water hammer pressure or for economic
reasons.
Please note that in the smaller pipe sizes, this
velocity limit permits the use of friction loss rates
higher than 4 feet per 100 feet.
Fluid velocity should be above 1-1/2 to 2 feet per
second in order to carry entrained air along to a
high point in the system where it can be purged.
It is generally accepted that if proper air control
is provided to eliminate air and turbulence in the
system, the maximum flow rate can be
established by a piping friction loss rate of 4 feet
of water per 100 feet. This allows velocities
greater than 4 feet per second in pipe sizes 2
inches and larger.
As piping ages, friction losses increase. It is
recommended that for most commercial design
purposes a safety factor of 10 to 15 % be added
to the values in the tables.
What is a “foot of water”? A column of water at
60°F, 5 feet tall, creates a constant pressure of 5
feet of water at the bottom of the column. If the
water column is 2.31 feet tall, the mass of water
creates a constant pressure (head) of one (1)
psi (pound per square inch). Pressure losses are
expressed either in “feet of water” or in psi.
Pump manufacturers usually prefer feet of water
units.
3.4.9.4 Pump or circulator selection
Performance characteristics of centrifugal
pumps are described by pump curves, which
plot flow versus head or pressure together with
other information such as efficiency and power.
Consult the manufacturer’s pump curves to
select the proper model or ask your pump dealer
or your HVAC wholesaler for a recommendation.
