Modern
HVAC systems, in theory, can satisfy the most demanding
requirements for indoor climate and operating costs.
However, in real life systems, not even the most sophisticated
controllers always perform as promised. As a result,
comfort is compromised and operational costs are higher
than expected. Among the common problems that appear
are:
• Some rooms never reach the desired temperatures,
particularly after load changes.
• Room temperatures keep swinging, particularly
at low and medium loads even though the terminals
have sophisticated controllers.
• Although the rated capacity of the production
units may be sufficient, design capacity can not be
transmitted, particularly during start-up after weekend
or night set back.
Fig
Nº40. An
external disturbance has the same effect on
each terminal in the module..
Click on image
to enlarge
These problems frequently occur because
incorrect flows keep controllers from doing their job. Controllers
do their tasks efficiently only if design flows prevail
in the plant and operating under design conditions. The
only way to get design flows is to balance the plant. Balancing
with manual balance valves gives the possibility to detect
most of the hydronic problems and to determine the pump
over sizing. The pump head can be adjusted at the correct
value, optimizing both the initial and operating pumping
costs. Manual balance valves perform itself as a flow-measuring
device, a measurement instrument and a balancing mean all
in one. Such features add valuable tools to obtain the correct
flow at design conditions throughout the plant.
Theory
and practice In theory it is sufficient
with one balance valve per terminal to appropriately
distribute the flow through the system. This requires
a correct calculating of the preset value for all
balance valves according to the current hydraulic
layout. If one or several flows are changed, all other
flows will be more or less affected. It may require
a long and tedious series of corrections to get back
to the correct flows. In practice, it is necessary
to divide larger systems into modules and installing
balance valves in such a way that readjusting only
one or few balance valves can compensate a flow adjustment
anywhere in the system. Proportionality law The terminals in the figure
40 form a module. A disturbance external to the module
causes a variation in the differential pressure across
A and B. Since the flow depends on the differential
pressure, flows in all terminals change in the same
proportions.
The flow through these terminals can therefore
be monitored through flow measurement in just
one of them, which can serve as a reference.
A balance valve common to all terminals can
compensate for the effect of the external disturbance
on the terminal flows in the module. This valve
is known as compensation valve. However, direct
return is the most common practice connecting
terminals in a module as in figure
41.
Fig.
Nº41. A
branch with several terminals and a compensation
valve forms a balancing module.
Click on
image to enlarge
The water flow through each terminal
depends on the differential pressure between A and
L. Any modification of this pressure affects the flow
in each terminal in the same proportion. But what
happens if we create a disturbance that is internal
to the module, for instance by closing the balancing
valve of terminal 3?
This will strongly influence the flows in pipes CD
and IJ, and thus the pressure loss in these pipes.
The differential pressure between E and H will change
noticeably, which will affect the flows in terminals
4 and 5, in the same proportion.
The fact that terminal 3 have been
closed has little effect on the total flow through AB and
KL. The pressure losses in these pipe lines change very little.
The differential pressure between B and K is changed
only somewhat and terminal 1 will not react to the
disturbance in the same proportion as terminals
4 and 5. Thus, the law of proportional flow change
does not apply for internal disturbances as shown
in figure 42. Optimum balancing
Figure 43 shows two
modules. The numbers indicate design pressure loss
in each terminal and the pressure loss in each balance
valve. Both modules are balanced. In both cases
the differential pressure on each terminal is the
required to obtain design flow through it. The pressure
losses are differently distributed between the balance
valves of the terminals and compensation valve.
Which balancing is the better of the two?
Optimum balancing means two things: (1)
that the authority of the control valves in maximized
for exact control, and that (2)
pump over sizing is revealed so that pump head and
thereby pumping costs can be minimized.
Fig.
Nº42. An
internal disturbance do not change the flow
proportionally in each branch..
Click on image
to enlarge
Fig. Nº43.
A set of terminals can
be balance in many ways, but only one is the
optimum.
Click on image
to enlarge
The
authority of a valve is based on the relationship
between the available differential pressure when the
valve is fully open (Delta P min) and when it is fully
closed (Delta P max). When the valve is about to close
the real flow is higher than the theoretical since
the differential pressure across the valve is grater
than the differential pressure when it is fully open.
The valve theoretical characteristic is then distorted.
The degree of this distortion is evaluated by the
ratio Delta P min / Delta P max.
Optimum balancing is obtained when the smallest possible
pressure loss is taken in the balance valves of the
terminals although a minimum pressure loss of at least
2 ft must be kept across the valve to allow precise
flow measurement.
Any remaining excess pressure is
taken in the compensation valve. The compensation valve
reveals the excess of differential pressure. The pump speed
for instance can be decreased correspondingly and the compensation
valve reopened. In example "b", the pressure drop
in the compensation valve and the pump head can be reduced
both by 10 ft, decreasing the pumping costs by 24 %.
Balancing
Variable Volume/Variable Speed Systems (VV/VS)
In constant volume systems, the terminal sub-circuits
are balanced against each other, so the same head
loss at design flow is maintained for all sub-circuits.
However, VV/VS systems demand another balancing strategy.
The reason for this follows: Figure 44 illustrates
a direct return system conventionally balanced when
constant speed (C/S) pumps are used. Each coil shows
its head loss (control valve head loss included) at
design flow, and each supply and return pipe section
has 4 ft head loss at the same condition.
Fig.
Nº44. Balance
valve setting for C/S pump and for direct return
systems.
Click on image
to enlarge
Now assume that a variable speed (V/S) pump is to
be applied to the balanced circuit shown in figure
44. The differential pressure sensor points
are taken across the "far" circuit, and
control is set for a maintained 25 ft. differential
?P at this point. The system will operate satisfactorily
at full flow, but operational difficulties may occur
at low loads, as shown in figure 45 at 50 % flow.
Fig.
Nº45. "Balanced"
system shown previously, converted
to V/S, and operating at 50 % flow
Click
on image to enlarge
Fig.
Nº46. "Better"
balance valve head loss settings; using
V/S pumping with piping systems at 50
% flow..
Click on
image to enlarge
In figure 45, the coil
1 can not get its full design flow for the 50 % flow
condition illustrated because it has a total design
flow resistance of (39' + 20') or 59', but only has
an available head of (36' –1') or 35'. Figure 46 illustrates
a better balance valve setting for the direct return
system using V/S pumping. The system is again shown
at 50 % flow. Now full flow capacity is available
at all sub-circuits and they are balanced against
each other so that each has an equal head loss; in
this case 25 ft at design flow.
Fig.
Nº47.
Balancing the terminals ensuring enough
differential head across each one,
minimizes circuits overflowing at
full system flow condition and allow
rated flow will be attainable at part
load.
Click
on image to enlarge
Since
piping head loss is not taken into account,
the direct return V/S system may be subject
to uneven pick-up after weekend shut-down.
High load operating costs may also be grater
for the direct return system compared with
reversed return, because a wide-open control
valve close to the pump may "steal"
more than its required flow share. For example,
at full system design flow, the coil 1 in
figure 46 will have 59 ft. differential head
across its circuit, but has only 27 ft head
loss at design flow. With a wide open control
valve, flow would increase by a factor of
(59/27)1/2 or about 48 % over design. Coil
2, coil 3 and coil 4 will be also overfed
about 37 %, 26 % and 14 %, respectively. Another
strategy would be balancing the terminals
in order to ensure enough differential head
across each coil to allow full flow through
it when the total system flow is halved.
The figure 47 shows such strategy that means again
overflow in the closer circuits at full system flow
condition but the amount of overflowing is lower than
before.
Now the coil 1 will receive 30 % ((59/35)1/2) over
design flow at nominal system flow condition, coil
2: 24 %, coil 3: 18 % and coil 4: 10 %.
