Primary-secondary
pumping is a tool HVAC engineers have been using for
nearly 50 years to accomplish the objective of designing
the most cost-effective pumping system while meeting
the client’s needs. The ASHRAE/IES Standard
90.1-1989 User’s Manual “highly recommends”
primary-secondary pumping for “systems with
large, high pressure drop distribution systems such
as those serving campuses and airports”. Primary
circuit is the place where chilled water is produced
and its principal component is the chiller. The secondary
circuit is responsible for the distribuition of the
chilled water to the terminal units. Among the components
of the secondary circuits are pumps, terminal units
such as fan-coils and air handling units (AHU) and
control valves.
Fig Nº1.
Common pipe design
principles.
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image to enlarge
Primary-secondary pumping is simple
in theory as well as operation. It is based on a simple fact:
when two circuits are interconnected, flow in one will not
cause flow in the other if the pressure drop in the piping
common to both is eliminated. To ensure a low pressure drop
(less than 1.5 ft) on the common pipe the length of this pipe
should be kept very short, and the supply and return tees
to the secondary circuit should be a maximum of three pipe
diameters apart. By keeping the pressure drop very low, water
that is flowing in the primary loop will not flow into the
secondary circuit until its circulator turns on. That way
the hydraulic isolation between both primary and secondary
circuit is achieved (Fig Nº1)
.
IIn the primary circuit the chilled water produced
circulates thanks to pumps installed generally in
tandem with each chiller. A separate circulator is
installed in the secondary circuit to establish flow.
This circulator is sized to move the flow rate and
to overcome the pressure drop of its circuit only.
The circulator should be located so it is pumping
away from the common pipe and discharging into the
secondary circuit. This causes an increase in pressure
in the secondary circuit rather than a reduction in
pressure which would occur if the pump were located
on the return pumping towards the common pipe.
The Primary-secondary pumping provides the means for
the constant volume pumping of the low horsepower
primary pumps through the chiller. These pumps are
lower horsepower than the secondary pumps because
they only have to overcome the friction loss associated
with the chiller, pipes, and valves in the primary
loop. The chiller pumps are balanced to the design
flow rate. Pump impellers should be trimmed to minimize
the pressure drop across the valve on the discharge
of the pump. This is an important energy saving measure.
The secondary pumps, in contrast, are higher horsepower
because they must overcome the friction loss associated
with the secondary loop: the distribution piping,
fittings, valves, coils, etc.
The volume of water circulating in the secondary circuit
is constant at all times because of the three way
control valve on each terminal unit of the system.
When no chilled water is needed in the terminal unit
the three way control valve opens its by-pass way
returning chilled water without entering the coil
of the terminal unit. Even in the extreme case of
chilled water inlet way of all terminal units were
closed, the volume of chilled water flowing in the
secondary circuit would remain constant due to the
opening of an alternative path (by-pass way) on each
control valve. When there is no mean, as an inverter,
that allow the variation of the pump speed rotation,
it would operate at its rated speed at all times.
That is why such systems are called constant volume/constant
speed (Fig. Nº2).
Most chilled water systems are design so that
secondary supply temperature equals primary
supply temperature. This requirement leads
to a simple design rule: primary circuit flow
should equal or slightly exceed secondary
circuit design flow rate .When the primary
flow equals secondary flow there is no chilled
water flowing through the common pipe and
the supply and
return
temperatures of both circuits are also equal (Fig.Nº3).
However such situation corresponds only to the full
load condition. At part load when one or two chillers
go off, the primary flow falls below the secondary
flow.Whenever the flow is greater in the distribution
loop than in the production loop, the excess flow
in the common pipe is in the direction towards the
secondary pumps. The result will always be a blending
of the return water with the supply water at a temperature
higher than what the chiller produces
(Fig. Nº4). The outcome is terminals receiving
supply water at a higher temperature and it could
mean a loss of humidity control in the zones. A higher
supply water temperature must be considered during
the coil selection process.
Fig. Nº3. Secondary
flow equals primary flow.
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image to enlarge
Fig. Nº4.
Secondary flow grater
than primary flow.
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Other
options can be considered. For example, chiller temperature
reset can be employed. Within the limits of the type
of machine, chiller temperatures can be reset to a
lower temperature to compensate for the increased
load and secondary flows. In essence, more capacity
is provided at a lower operating efficiency. The increase
in cost of chiller operation due to the lowering of
the chiller supply temperature can range from 1 to
3 percent per degree of reset. This is a very desirable
alternative, especially when large chillers are in
use. The longer the start of a lag chiller can be
delayed, the better it will perform when it is finally
brought on line.
If
a small portion of the load requires a fixed temperature,
a small chiller in series with the load may also be
considered.Many chilled water systems, however, and
most hot water, can be design so that secondary circuit
design supply water temperature is different from
the primary water supply temperature. The Fig.
Nº5 illustrates one of the outstanding
advantages of primary-secondary pumping; that the
primary circuit can be designed for very high temperature
drops so that the primary flow and consequently pumping
horsepower and pipe cost can be reduced. It should
be emphasized that the primary circuit serves only
a heat conveyance function and that its temperature
drop or rise can be entirely different from the secondary
circuit it serves.
Fig.
Nº5. Primary-secondary
system with the primary circuit designed with
a high temperature drop.
Click on
image to enlarge
In general, the system design where secondary circuit
flow rate is grater than primary provides smooth reset
controllability, allows deep primary circuit temperature
drops and can be used to grate advantage in the numerous
primary-secondary control arrangements made possible.
Actually, the most of the primary-secondary designs
establish that the primary circuit flow will be equal
to or less than secondary flow. It means that in such
systems the return temperature of the primary circuit
will always be equal to secondary return.
