As water flows through the
pipes and radiation, it “rubs”
against the pipe wall causing frictional
resistance. This resistance
can affect the performance of the
heating system by reducing the
desired flow rate from circulating,
thus reducing the heating capacity
of the system. By knowing what
this resistance will be, you can select
a circulator that can overcome
the system’s pressure drop.
Typically, in today’s systems, we
use “feet of head” to describe the
amount of energy needed so that
the required GPM is delivered
out to the system. There are pipe
sizing charts that have calculated
the pressure drop in foot head
of energy loss for any flow rate
through any size pipe. There are standard piping practices
in which the industry references that limit the amount
of GPM for a given pipe size.
This is based on two reasons:
1. Velocity (how fast the water is moving
inside the pipe) can create noise concerns,
and in extreme conditions, erosion problems.
2. The required head loss can become
so excessive that the required circulator’s
head capacity makes for a very “unfriendly”
system selection, which can lead to control
valve and velocity noise problems (the industry
standard is to select a pipe that offers
the frictional resistance between 1'–4' for
every 100' of piping).
For more than 50 years, Bell & Gossett
has provided a tool for the hydronics industry
called The System Syzer. It is very useful
in calculating GPM, the proper pipe size to
support the GPM and the corresponding
pressure drop and velocity for the calculated
GPM.
When calculating gallons per minute
(GPM), don’t forget about water
If you have any questions or comments, e-mail me at
gcarey@fiainc.com, call me at FIA 1-800-423-7187 or
follow me on Twitter at @Ask_Gcarey. ICM
pressure.
ICM/July/August 2020 17