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With the advent of less expensive and more reliable variable
frequency drives, primary pumped constant volume chilled water
and hot water systems designs evolved into primary-secondary
pumped systems that use variable frequency drives to vary the
speed and thereby HP of the secondary (distribution) loop pumps
according to demand. In theory, pump energy is reduced as only
the required water is distributed in lieu of 100% of the design
water as was the case in constant volume systems.
The vast majority of primary-secondary systems designs share
these common characteristics:
- Full size, usually front, decoupler
- Typically 1.5 ft. maximum pressure drop in the decoupler
- Decoupler 3 pipe diameters minimum length
- Decoupler typically same size as distribution piping
- Matched primary/secondary flow rates
- Differential pressure controls the secondary flow rates
This piping configuration can produce three possible flow conditions:
Primary (production) flow equal to secondary (distribution)
flow. While ideal, it begs the question why utilize this piping
design? Why wouldn't a variable flow primary only system be
cheaper to install and more energy efficient to operate? The
simple answer is yes it is, and Energy-Environment-Economics
has converted
existing constant volume systems to variable primary,
has designed
new variable primary systems to replace existing constant volume
systems, and has commissioned newly installed variable
primary systems.
The practical answer is that prior to widespread use of Direct
Digital Controls (DDC) on water chillers, chiller manufacturers
were uneasy about varying flow rates thru their machines. Hence
the constant volume primary pumps. Today's chillers are comfortable
with varying flow rates, as long as the rate of change is not
extreme, so variable primary systems are becoming more popular.
Another possible hydronic condition......
In this flow condition the building's load requires more water than a single
chiller system can deliver, and the excess water is decoupled to the return
loop, depressing the chillers' entering water temperature and reducing load.
Note that the secondary pump variable frequency drives have slowed to provide
only the necessary GPM. While acceptable but not optimal, it certainly is
preferable to....
Known as "reverse" decoupling, this flow condition
raises the delivered chilled water temperature, causing the
chilled water coils to require more water and thereby causing
the pumps to increase flow rates and consumed HP. The obvious
solution is to start another chiller system and return to flow
condition #2 described above. The real problems arise when the
system temperature differential (DT) is less than design; the
chillers are unloaded but are required to run because the loads
are requiring more water.
For example, the formula for coil capacity is QBtu/H = (GPM*DT)*500.
Obviously for the same capacity the GPM would have to be doubled
if the DT was halved. Most of today's systems are operating
with less than design DT's, necessitating additional flows usually
resulting in starting additional chiller systems to provide
the required flow rates. For example, from Figure 4 above and
for a constant cooling load, a five-degree system DT would require
3000 GPM in lieu of 1500 GPM, so an additional chiller system
- chiller, primary pump, condenser water pump, and cooling tower
- would be forced to run. And, of course, the secondary pump
variable frequency drives must speed up to increase to the required
flow rate. Low system DT's are an energy efficiency cancer.
Energy-Environment-Economics can diagnose why
your system is suffering and can devise solutions to alleviate
the condition and save large amounts of energy and money.
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