Page 18 - Oil and Energy January 2013

18 • OIL&ENERGY
Sizing a Circulator Just Takes Some Math
By Bruce Marshall, Emerson-Swan
THE DICTIONARY DEFINES A SYSTEM AS A
group of interacting, interrelated, or inter-
dependent elements forming a complex
whole. When designing and installing a
hydronic heating system, we are in fact
creating a group of elements that interact,
that are interrelated and are interdependent,
and they do form a very complex whole
that is designed to keep people comfortable
while minimizing energy usage.
The system’s relative success or failure
depends on how well these elements work
together to perform the system’s stated
function. The definition of hydronics is the
science of transferring a definitive amount of
BTUs from a source device to a heat transfer
device and back via the movement of water
or solution thereof. A key component of a
modern hydronic system is the circulator
and its main function is to move heated
water (BTU/HR) through a distribution
system (the radiators) and back.
BUILDING HEIGHT IS NOT A FACTOR
It’s important to remember than when
sizing a circulator, you do not need to take
into account the height of the building. The
physical height of the building does NOT
equal the feet of head. Part of defining a
circulator as opposed to a pump is the fact
that we are in a closed loop system versus
an open system. Unlike a closed system, an
open system has to overcome static head
as well as pressure drop. Examples of this
would be a well or a sump pump system.
The circulator does not need to lift the
water to the top of the building due to the
simple fact that what goes up must come
down. The circulator doesn’t have to lift
the water to the upper floors – the weight
of the water coming back down the return
side is a counterbalance. Think of the
circulator as the motor on a Ferris wheel.
The motor doesn’t have to lift the weight
of the people up – there are people on the
other side of the wheel coming back down.
All it has to do is overcome the friction
loss of the bearing assemblies in the wheel.
A circulator doesn’t have to lift the water –
it only has to overcome the friction loss – or
head loss – of the system.
All piping systems impart friction loss
on the fluid in the system, and under-
standing this is key to making sure your
hydronic system functions properly. If you
do the math, calculating the flow require-
ments for circulator is pretty simple: It’s
basic arithmetic. Calculating the “other”
half – head pressure (or friction loss) – is a
little tougher. Use the Universal Hydronics
formula to determine how much flow
capacity the circulator needs.
GPM = BTUH ÷ Ƌ T x 500
GPM is gallons per minute. BTUH
is the calculated system load. Ƌ T is the
temperature difference across the system
at design conditions and we use 20° F for
our systems. 500 is a constant – it is the
weight of a gallon of water (8.33 lbs.) times
60 minutes. When we have determined the
load of the system all we need to do then is
to divide by 10,000 (20 x 500) and we have
our GPM requirement for the circulator.
As an example, let’s say we are zoning with
circulators and have a 30,000 BTU zone of
baseboard or 50 feet of element. When we
divide the 30,000 by 10,000 we determine a
flow rate of 3 gallons per minute.
CHOOSING THE PIPE SIZE
What size pipe should we use for this
zone? Well, the guidelines for pipe sizing
are as follows:
• 2 to 4 gallons per minute of flow,
use ¾” M copper;
• 4 to 8 GPM, use 1 inch;
• 8 to 14 GPM, use inch and a quarter;
• 14 to 22 GPM, use inch and a half.
These all fall within hydronics guidelines
for pipe sizing and keeping flow velocities
at no less than 2 feet per second and no
more than 4 feet per second. At velocities
greater than 4 feet per second, the system
will produce velocity noise – soon followed
Hydronics