The variable
volume/variable speed (VV/VS) scheme is a variation
of the constant volume/constant speed (CV/CS) design
with a grater energy efficient performance (Fig.
11). Basically, the
VV/VS scheme differs from the CV/CS as to the way
the system tracks thermal load. In CV/CS systems the
volume of chilled water circulating is the same at
all times and thermal load variation is reflected
in system delta T fluctuation.
On the other hand, in VV/VS systems the flow will
track the load allowing considerable energy saving
in pumping at part load.
Fig. Nº11VV/VS primary-secondary
pumping system.
Click on
image to enlarge
To achieves such goal the three
way valves that always bring an alternative "path"
to the flow at part load condition, must be replaced
by two way valves. The secondary pumps will now be driven
by VSD that allows variable speed operation according
to flow terminals demand.
The control loop closes with a differential pressure
sensor that must be appropriately situated in the
hydraulic circuit. Such device will send a sign
proportional to the increase of the pressure in
the zone of the circuit that is being monitored.
A differential pressure setpoint will then be fixed
in order to allow enough flow in the monitored zone
at all load conditions. At part load, when two way
valves modulate to close, the differential pressure
setpoint will then be maintained by a proportional
reduction of the pump speed that means system flow
reduction. The pumping power consumption is a cubic
function of flow change so a 50 %
speed reduction leads to an 87.5 % of energy saving.
Operating a VV/VS primary-secondary pumping system
can lead to three flow conditions that are:
Secondary circuit flow equals
primary circuit flow: This is an uncommon
flow condition because it means the system is completely
balance which is rarely to find in real life systems
(Fig. 12). However,
it is worth analyzing such a flow condition in order
to achieve a better knowledge of the system operation.
The constant volume that circulates through the
secondary circuit is equal to 1000 GPM, the chilled
water supply (CWS) temperature is 45 F with a temperature
rise of 10 F. The primary circuit is equipped with
three equal chillers of 415 Ton each one. The flow
that circulates through the active chiller equals
415 Ton x 24 x 1/10 F = 1000 GPM. So, when only
one chiller is on, primary flow equals secondary
flow, and no water circulates through the common
pipe. At this moment the thermal load is 415 Ton.
The remainder of the time of the secondary
or load flow will be greater or less than
the primary flow. The moment the load increases
in the space, the two-way valves begin modulating
to a more open position and deliver additional
chilled water to satisfy the additional load.
When this occurs, the pressure and flow relationship
changes such that the distribution flow is
greater than the production flow
(Fig. 13).
Fig. Nº12Secondary flow
equals primary flow.
Click
on image to enlarge
Secondary circuit flow grater than primary circuit
flow: In this case the thermal load has increased
from 415 Ton to 500 Ton. The flow through the active
chiller remains the same (1000 GPM) but the secondary
flow has risen to 1200 GPM due to a more open position
of the two way valves. Such a secondary flow increase
is a reaction of the VSD that steers the pump aimed
to keeping the differential pressure setpoint. To
balance the mass flow, the excess 200 GPM must run
through the common pipe. The temperature of the 200
GPM in the common pipe is 55 F. This blends with the
1000 GPM of 45 F supply water, resulting in 1200 GPM
of 46.7 F blended supply water
Fig. Nº13.
Secondary flow grater than
primary flow.
Click on
image to enlarge
A higher supply water temperature
profoundly reduces cooling coil capacity, especially latent
cooling capacity. One solution could be considering higher
supply water temperatures during the coil selection process
at the beginnings of the projects. Other options as chiller
temperature reset can be considered. 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.
Primary circuit flow grater than secondary circuit
flow: When thermal load keeps rising up to
625 Ton and another chiller is brought on line the
third flow condition appears (Fig. 14). The primary
circuit flow is 2000 GPM and through the secondary
circuit circulates 1500 GPM. The 500 GPM excess must
flow through the common pipe at the chilled water
supply temperature of 45 F and blends with the 1500
GPM secondary chilled water return to produce 2000
GPM at a reduced entering chilled water temperature
of 52.5 F. All chilled water returning from the loads
is blended prior to reaching the chillers. All of
the chillers on line will therefore be receiving the
same temperature water at their return.
Fig.
Nº14. Primary
flow grater than secondary flow.
Click on
image to enlarge
When the system
is piped in this manner, the chillers will always
be equally loaded. Furthermore, the chillers will
always be subjected to their design flow at an equal
temperature. Because the chillers are receiving their
design flow rate at 52.5 F rather than the design
temperature of 55 F, the chillers will be “unloaded”
at the ratio of: (1- (55 F –52.5 F)/(55 F –
45 F))*100 = 75 %
The origin of the three flow conditions analyzed lies
in the different between the linear nature of the
thermal load and the stepped way the chiller plant
acts to track it. As the two-way valves modulate in
response to a varying load, the flow follows directly.
The more chillers in the plant, the smaller the steps
to track the load. The addition of chillers in a variety
of sizes makes the incremental steps smaller. The
additional chillers and variety of chiller sizes can
produce a curve that is nearly linear. When the chiller
plant is designed to produce a near-linear flow function,
the flow in both directions through the common pipe
and the consequences of blending supply with return
chilled water are minimized. Increasing of supply
water temperature then lasts for a shorter duration
which is a very critical factor in design regions
where humidity control is a concern.
