THERMO 2000 VOLTMAX 23, Use & Care Manual

Page: 13 / 38

TURBOMAX Use and Care Manual with Installation Instructions July 2014 Page
13
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 larger pipe, however, costs more initially
and must be balanced against the cost savings
realized by a 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 hydronic 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 allows 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 with
the flowing water to the point of air separation
where the air can then be separated from the
water and purged.
Velocities greater than 4 feet per second are
often used on piping larger than 2 inches.
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 the piping friction loss rate at 4
feet of water per 100 feet. This allows the use of
velocities higher than 4 feet per second in pipe
sized 2 inches and larger.
Tables 1 and 2 provide friction loss values for
new pipe. It should be taken into consideration
that as the pumping system 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 mass of water at
60°F standing inside a vertical column or a
vertical pipe 5 feet tall creates a constant
pressure of “5 feet of water” at the bottom of the
column or pipe. If the water column is 2.31 feet
tall, the mass of water creates a constant
pressure (head) of one (1) p.s.i. (pound per
square inch). Pressure losses are expressed
either in “feet of water” or in p.s.i.. Usually,
pump manufacturers use feet of water units.
Pressure loss from water flow in pipes
To get the proper boiler water flow through the
water heater tank, the pump must overcome the
pressure loss from water flow in pipes, valves,
fittings and components in the piping circuit.
Friction is a resistance to flow.
Pressure loss (or pressure drop) from friction in
straight pipe, in Tables 1 and 2, are in feet of
water per 100 feet of pipe length.
Example 1: What is the pressure drop caused by
water flowing at a rate of 25 GPM (gallons per
minute) through a 1-1/2 inch diameter copper
tube which has a length of 100 feet?
Answer: by looking at the data in Table 1, you
notice that the pressure loss is 5.46 feet of water
for a tube of 100 feet.
Example 2: What if the pipe has a length of 40
feet for the same flow rate?
Answer: the pressure loss is 40% (= 40 feet /
100 feet) of 5.46 feet of water per 100 feet. The
calculated pressure loss is 2.18 feet of water (=
40% X 5.46 ft water).