Variable flow pipework systems

27 Pages·2006·0.57 MB·English

by  Butcher, Ken, Carwardine, Helen, Parsloe, C

Variable flow pipework
systems
CIBSE Knowledge Series: KS7
Principal author
Chris Parsloe
Editors
Helen Carwardine
Ken Butcher
CIBSEKnowledge Series — Variable flow pipework systems
The rights of publication or translation are reserved.
No part of this publication may be reproduced, stored in a retrieval system or transmitted
in any form or by any means without the prior permission of the Institution.
© July 2006 The Chartered Institution of Building Services Engineers London
Registered charity number 278104
ISBN-10:1-903287-77-4
ISBN-13: 978-1-903287-77-4
This document is based on the best knowledge available at the time of publication.
However no responsibility of any kind for any injury, death, loss, damage or delay however
caused resulting from the use of these recommendations can be accepted by the
Chartered Institution of Building Services Engineers, the authors or others involved in its
publication. In adopting these recommendations for use each adopter by doing so agrees to
accept full responsibility for any personal injury, death, loss, damage or delay arising out of
or in connection with their use by or on behalf of such adopter irrespective of the cause or
reason therefore and agrees to defend, indemnify and hold harmless the Chartered
Institution of Building Services Engineers, the authors and others involved in their
publication from any and all liability arising out of or in connection with such use as
aforesaid and irrespective of any negligence on the part of those indemnified.
Typeset by CIBSEPublications
Printed in Great Britain by Latimer Trend &Co. Ltd., Plymouth PL6 7PY
In systems using DPCVs to control pressure, such as that shown in Figure 5,
constant flow regulators can be used instead of fixed orifice double regulating
valves to maintain an accurate flow distribution under all operating
conditions. However, it should be remembered that a constant flow regulator
does not remove the need for the upstream DPCV. This is because under
high pressure, low flow conditions (as might occur in a terminal branch when
its 2-port valve was closing) a constant flow regulator would move fully open
as it attempted to restore the flow to its design value. This would leave the
2-port valve unprotected as it shuts off against the full branch pressure. If the
valves have an on/off characteristic, then flow modulation is not a concern
but, without any form of upstream pressure control, the valve might still
generate some noise or cavitation over the final part of its travel.
References
1 Building control systems CIBSE Guide H (London: Chartered Institution of Building Services
Engineers) (2000)
2 Reference dataCIBSE Guide C (London: Chartered Institution of Building Services Engineers) (2001)
Bibliography
Parsloe C J The commissioning of water systems in buildings BSRIA Application Guide AG 2/89.3
(Bracknell: Building Services Research and Information Association) (2002)
Parsloe C J Variable speed pumping in heating and cooling circuits BSRIA Application Guide AG14/99
(Bracknell: Building Services Research and Information Association) (1999)
Petitjean R Total hydronic balancing(Ljung, Sweden: Tour and Anderson AB) (1994)
Teekaram A and Palmer A Variable-flow water systemsBSRIA Application Guide AG16/2002 (Bracknell:
Building Services Research and Information Association.) (2002)
Water distribution systemsCIBSE Commissioning Code W (London: Chartered Institution of Building
Services Engineers) (2003)
22 CIBSEKnowledge Series — Variable flow pipework systems
In systems using DPCVs to control pressure, such as that shown in Figure 5,
constant flow regulators can be used instead of fixed orifice double regulating
valves to maintain an accurate flow distribution under all operating
conditions. However, it should be remembered that a constant flow regulator
does not remove the need for the upstream DPCV. This is because under
high pressure, low flow conditions (as might occur in a terminal branch when
its 2-port valve was closing) a constant flow regulator would move fully open
as it attempted to restore the flow to its design value. This would leave the
2-port valve unprotected as it shuts off against the full branch pressure. If the
valves have an on/off characteristic, then flow modulation is not a concern
but, without any form of upstream pressure control, the valve might still
generate some noise or cavitation over the final part of its travel.
References
1 Building control systems CIBSE Guide H (London: Chartered Institution of Building Services
Engineers) (2000)
2 Reference dataCIBSE Guide C (London: Chartered Institution of Building Services Engineers) (2001)
Bibliography
Parsloe C J The commissioning of water systems in buildings BSRIA Application Guide AG 2/89.3
(Bracknell: Building Services Research and Information Association) (2002)
Parsloe C J Variable speed pumping in heating and cooling circuits BSRIA Application Guide AG14/99
(Bracknell: Building Services Research and Information Association) (1999)
Petitjean R Total hydronic balancing(Ljung, Sweden: Tour and Anderson AB) (1994)
Teekaram A and Palmer A Variable-flow water systemsBSRIA Application Guide AG16/2002 (Bracknell:
Building Services Research and Information Association.) (2002)
Water distribution systemsCIBSE Commissioning Code W (London: Chartered Institution of Building
Services Engineers) (2003)
22 CIBSEKnowledge Series — Variable flow pipework systems
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
2 Calculating pump energy savings . . . . . . . . . . . . . . . . . . . . . . . . . . .2
3 Sizing control valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6
3.1 Cavitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8
4 System design options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
4.1 Self-balancing layouts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
4.2 Differential pressure control valves (DPCVs) . . . . . . . . . . . . . . . .16
4.3 Constant flow regulators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22
Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22
CIBSEKnowledge Series — Variable flow pipework systems
1 Introduction
This publication explains how to design re-circulating heating or cooling
water systems incorporating variable speed pumps. Correctly designed, these
systems have the potential to deliver worthwhile energy savings over the
lifetime of a building.
Most constant speed pumps operate with fixed flow rate and energy
consumption for their entire lifetime. When zones are satisfied, 3- or 4-port
valves divert heating or cooling water away from terminal units through by-
passes. Overall flow through the pump remains roughly constant.
In systems where pump speed is allowed to vary, the flow rate constantly
changes in response to demand. When zones are satisfied, 2-port control
valves close enabling the pump to reduce speed and save energy.
Since the mid-1990s variable speed pump drives have become a viable
alternative to constant speed drives. However, there has been much debate
regarding how to maximise their energy-saving potential. There are many
ways to make a system work with variable speed pumps. However, if not
designed correctly, the energy savings achieved may be small relative to the
increased cost of the system.
In order to be worthwhile, the design must:
— achieve a significant reduction in pump energy consumption (relative to
that for an equivalent constant flow system)
— incur a minimal increase in installed cost
— not reduce the effectiveness of internal space temperature controls.
With regard to the last point, it is important to remember that any savings in
pump energy will easily be negated if the control of internal space
temperatures is compromised. A variation of 1 °C from the internal design
condition is likely to result in a bigger energy penalty than any savings
achieved by the pump.
This publication begins by explaining the main issues relevant to any variable
flow design, including how to calculate pump energy savings, size control
valves (in systems requiring modulating control of water flow rates) and avoid
valve noise or cavitation in valves. It also presents two cost-effective solutions
for designing variable flow heating and cooling systems which should achieve
worthwhile pump energy savings without compromising the effectiveness of
temperature controls.
CIBSEKnowledge Series — Variable flow pipework systems 1
2 Calculating pump energy savings
For heating or chilled water systems serving air conditioning plant, the
requirement for maximum heating or cooling occurs only at start up, or on
infrequent peak design days. For the majority of the time, a reduced heating
or cooling output will suffice. During these periods there is potential to pump
less water, thereby reducing the annual pump energy consumption.
Pump energy can be saved because there is a useful correlation between
pump speed, pressure, flow rate and power. For any pump that is pumping
against a fixed resistance, the consequences of changing pump speed (from
N to N ) can be predicted from the pump similarity laws:
1 2
Q = Q (N / N )
2 1 2 1
Δp = Δp (N / N )2
2 1 2 1
P = P (N / N )3
2 1 2 1
where Nis the pump speed (rev/s), Qis the flow rate (m3/s), Δpis the
differential pressure across the pump (Pa) and Pis the pump power (W).
In other words, if pump speed is reduced to 25% of its previous value then:
— flow rate (Q) is also reduced to 25% of its previous value
— pump pressure generated (Δp) is reduced to 6.25% (i.e. one sixteenth
of its previous value)
— pump power consumption (P) is reduced to 1.6% (i.e. one sixty-fourth
of its previous value).
The same consequence can be seen when these relationships are applied to
the standard equation for determining pump power:
P= Δp Q / η
where ηis the overall pump efficiency (%).
It can be seen that if pump speed is reduced to 25%, causing flow to be
reduced to 25% and pump pressure to be reduced to 6.25%, then, as
predicted by the pump similarity laws, pump power reduces to 1.6% (i.e.
0.25 times 6.25).
2 CIBSEKnowledge Series — Variable flow pipework systems
This relationship holds true provided the pump is pumping against a fixed
resistance because, for this situation, pump efficiency usually remains fairly
constant regardless of changes in pump speed. Therefore, if the pipework
system is serving a uniform heating or cooling load then it should be possible
to keep the system resistance constant and regulate pump speed up and
down in response to demand, thereby achieving all of the 98.4% energy
saving predicted at 25% flow.
However, most systems serve multiple zones with variable loads each
requiring individualised control of terminal units. This control is typically
provided by 2-port control valves which modulate flow as required to suit
the zone. In a system with 2-port control valves, the overall system resistance
will not be fixed but will increase and decrease as valves open and close.
In this situation the actual pump energy savings achievable will depend on the
way in which pump speed is controlled. The easiest way to control pump
speed is to make it respond to a differential pressure signal between two
points somewhere in the system.
The best energy-saving options are:
— vary pump speed based on the pump differential pressure and using an
integral speed control characteristic designated by the pump
manufacturer
— vary pump speed to maintain pressure constant at system extremities
(using remote differential pressure sensors).
The consequences of each option in terms of pump and system resistance
characteristics are shown in Figures 1a and 1b. For each example, a minimum
system flow rate of 25% has been assumed.
It can be seen from Figure 1a that pump integral controllers are able to
generate their own speed control characteristics which determine how the
pump will respond to changes in system resistance. The pump operating
point will always lie somewhere on this characteristic. Figure 1a shows a
straight line control characteristic, but pump manufacturers can also provide
curved characteristics which give larger reductions in pump speed for the
same operating conditions.
Pumps controlled in this way have the advantage that they avoid the need for
remote differential pressure sensors. However, with all integral controllers
there is an assumption that the system has a fairly uniform and predictable
load pattern and that all 2-port valves will open and close roughly together. If
the load pattern is not uniform, i.e. some circuits are likely to remain fully
CIBSEKnowledge Series — Variable flow pipework systems 3
Figure 1a:
η
Varying conditions in a 1
Pump
η
system with pump efficiency 2
pressure controlled by
pump integral controller
System
characteristic
Reduced
p pump η
Δsure, Δp1 speed Oat pmearaxtimingu mpo lionatd ciency,
res effi
P p
m
u
Δp2 Pump P
Operating curve
Integral pump speed
point at
control characteristic
minimum
(All part load operating
load
points occur on this line)
Q Q
2 1
(= 0.25 Q) Flow rate, Q
1
Figure 1b:
η
Pump η1
Varying conditions in a
2
efficiency
system with pump
pressure controlled to
maintain constant System
characteristic
pressure at system
p Reduced η
extremities Δsure, Δp1 pump speed Oat pmearaxtimingu mpo lionatd ciency,
res effi
P p
m
u
P
Pump
curve
Part load operating
points occur anywhere
Δp2 in this region
Operating point
at minimum load
Q Q
2 1
(= 0.25 Q1) Flow rate, Q
open whilst the majority close down, then there is a risk that the fully open
circuits may be starved of flow as pump speed reduces.
The use of remote differential pressure sensors at system extremities is a
more precise way of controlling pump speed. Pump speed is controlled such
that the minimum design pressure is always available at each extremity.
Therefore, as shown in Figure 1b, the part load pump operating point could
lie anywhere within a range of values between maximum and minimum load
4 CIBSEKnowledge Series — Variable flow pipework systems
conditions. Multiple sensors are required because in a variable flow system
where 2-port valves may close down in random order, the system index may
not remain in one location but could move around to different parts of the
system.
It can be seen from Figure 1b that for a system controlled in this way the
minimum load operating point is not fixed by any pre-determined control
characteristic but is free to drop by as much as required. It is therefore likely
that the use of remote sensors will achieve larger energy savings than if
integral speed controllers are used.
For each pump speed control method, the pump energy saving achievable
between maximum and minimum load conditions will be equal to the
difference between maximum and minimum load pump power, i.e:
Pump energy saving = (Δp Q / η)– (Δp Q / η)
1 1 1 2 2 2
By plotting maximum and minimum load pressure loss and flow rate
conditions on the pump manufacturer’s pump curve, the change in pump
efficiency and consequent energy saving can be determined. However, to
complete this calculation, pump duties need to be estimated for both
maximum and minimum load conditions. This may require repeating the
pump sizing exercise.
CIBSEKnowledge Series — Variable flow pipework systems 5