1. Bulletin 1005-1
TechTALK #4 Engineering
correct heating and
cooling balance in
buildings
Hydronic System Design Update
Controlling
Differential Pressure
is the secret to perfect
room temperature in
variable flow systems
So why is Differential Pressure Control
essential in most variable flow systems?
Most hydronic systems today are variable flow with 2 way modulating control valves installed to
vary the flows to achieve the required heat loads. However, little is known or accepted on the effect
of changing differential pressure in the system.
The advantage of variable flow distribution systems is that you
save pump energy. The consumption of pump energy depends
on the product of flow and pump head.
Pump head x Flow
Pumping Costs C0 +
Pump Efficiency
The disadvantage of variable flow is that the differential
pressure across the control valves varies. The lower the flow,
the higher the differential pressure across the control valves.
Two (2) way modulating control valves are installed to vary
the flows to achieve the required loads. However little is
known or accepted on the effect of changing differential
pressure in the system. Balancing Valve
The modulating 2 way control valve creates a supplementary Terminal
pressure drop in the hydraulic circuit to limit the water Control Valve
flow to the required value. The water flow depends on the
differential pressure applied to the valve.
Above: The differential pressure applied to the control
valve depends on its degree of opening
As the control valve throttles (reducing the flow), pressures
reduce in all the static devices in the system, terminals,
pipes & accessories, with most of the increased pressure In this paper, we will discuss the effects of varying
then being applied to the only moving component: flows and pressure on the performance of the
the 2 way modulating control valve. system & equipment within the system.

2. The Need for Differential Pressure Control
Average cooling load [%]
Climate conditions affect
100
90
plant loadings 80
70
60
50
40
Australian conditions 30
20
75% (varies slightly by state) of the cooling 10
season in Australia plant loading requirements 0
will be 50% or less. 0 10 20 30 40 50 60 70 80 90 100
% of cooling season below the load
Load variations are heavily influenced by: °C 30
• Sunshine effects (up to 750 W/m² for a East 25
façade in December around 4pm at 50° South). 20
• Building occupancy (1 sitting person: ±110 W, 15
computers). 10
Seasonal Variation
5
Average Monthly Temperatures (degree C)
0
n h ril y us
t ct v c
un u
eb
ly
p
e
F ug N D
o
e
M
Ja
Se
c
Ap
a
O
ar
M
J
J
A
Figures supplied by the Melbourne Bureau
of Meteorology covering the year 2006.
Simulation
Balanced System
• 100% Load and Flow
Obtaining the correct design flow
This system consists of two (2) branches servicing five
(5) terminals on each brabch, modulating (2) way control
valves and balancing valves on each terminal.
Control Valve Authority is defined as:
Control Valve Authority b = ΔP across control valve at design flow
ΔP valve closed
Design Parameters
Terminal flow: 0.35 L/sec
Terminal ΔP: 30 kPa
Control Valve ΔP: (at design flow) 45.8 kPa
ΔP Control Valve closed: 105.3 kPa (measured on simulator)
Pump Head (VSD): 255 kPa
Example opposite, 100% load and flow
Control Valve Authority 0.43 (last terminal, first branch)
which is considered acceptable for good control.
However what happens when the control
valve modulates?
Computer simulation using TA-Simul
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3. Differential Pressure variations due to Load & Flow
• Cooling Emission ΔP piping
Systems 120% 100%
100%
operate at 80%
80%
50% load 60%
50 % 60%
or less for load 40%
40%
around 75% 20% 20%
4% press.
of the cooling 0% drop 0%
season! 0% 50% 100% 150% 200%
Flow
0% 20% 40% 60% 80% 100%
Flow
20 %
At constant supply
water temperature flow
The pump head applies itself almost
Pressure drops reduced to 4% entirely on the 2-way automatic
Balanced System of their initial value temperature control valves
• 50% Load, 20 % Flow
(Computer simulation using TA-Simul)
Example opposite: 50% load requires
only 20% flow, the frictional losses in the
pipework, terminal and all static devices reduce
to 1/25th or 4% of design (refer to graph above).
Even if the VSD or bypass pressure differential
sensors located in the main plant, maintains a
stable differential pressure at the start of the
system, the control valve pressure drop will still
increase dramatically as it throttles, absorbing
96% of the design pipework and terminal
pressure drops.
Distortion of
Control Valve Authority!
What does this mean?
The modulating 2 way control valve creates a
supplementary pressure in the hydronic system
to limit the flow to the required value.
In this example, the Load has reduced to 50% of
design which corresponds to a flow requirement
of 20% of design. The control valve has throttled
to 27% of its stroke to achieve the required flow Water Flow %
of 0.07 L/sec. The ΔP across the control valve has 100
90
increased from the design value 45.8 kPa to a 0.18 100% Load & Flow
80
massive 246.7 kPa! 70
Authority b = 0.43
This affects the control valve authority, and 60
0.43
performance from 0.43 to 0.18 - this control valve 50 50% Load, 20% Flow
40 Authority b = 0.18
is now in ON/OFF mode (refer to graph). 30
20
10 Control valve authority:
0 at design flow and 20% of
0 10 20 30 40 50 60 70 80 90 100 design flow
Valve Lift %

4. Perfecting Differential Pressure Control
Balanced System
WITH ΔP Controllers
• 50% Load, 20 % Flow
Differential pressure controllers have been
installed on each branch, maintaining a
relative constant ΔP on each riser.
In this example, the load has again reduced
to 50% of design corresponding to a flow of
20% of design.
The differential pressure controller in the
first branch has been adjusted to 97.7 kPa.
Irrespective of the upstream pressure
(250.9 kPa) the downstream pressure or set
pressure (97.7 kPa) ΔPL (within tolerances)
will be maintained ensuring ΔP across the
control valve does not exceed ΔPL (97.7 kPa).
In this instance the Authority of the control
valve has improved.
Control Valve Authority of:
0.47 (last terminal, first branch)
with differential pressure controllers
installed.
Stroke of the control valve is also improved
to 41% of opening.
Conventional Balanced System
with ΔP Controllers - 50% Load & 20% Flow
(Computer simulation using TA-Simul)
Water Flow %
100
90 100% Load & Flow
0.18
80 Authority b = 0.43
70 0.43 Balanced System
60
0.47 50% Load, 20% Flow
50
Authority b = 0.18
40 Balanced System
30
20 50% Load, 20% Flow
10 With ΔP control
Authority b = 0.47
0
0 10 20 30 40 50 60 70 80 90 100
Valve Lift h%
The STAP has been set to deliver 97.7 kPa (ΔPL).
Any changes in ΔPH the valve will automatically re-adjust
itself to maintain the downstream pressure ΔPL.
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5. Energy Savings & Improved Performance
ΔP control assists in achieving
Green Star Ratings
Case study: Asian University Test conditions: 50% Load and 20% Flow.
Tests were carried out on STAP ΔP controller The first test was carried out with the capilliary pipe
and STAD balancing valves servicing an AHU to the STAP ΔP controller disconnected, disabling the
at the University, logging the measured ΔP controller from the circuit. Continuous logging of
flow, return water temperature and power measurements were taken at 30 minute intervals for 7
consumption. days.
Before each test, the system was balanced at Tests were repeated over the same period in time with
design flow, the STAP ΔP controller was tuned the ΔP controller connected, continuously logging the
to maximise energy usage. flow, return water temperature and power consumption.
The results in the graph shown here were remarkable:
Graph drawn from actual measured data over one day
during the summer months
Flow Rate L/sec Without ΔP control With ΔP control
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
20% of design Flow
0
Time .00 .30 .00 .30 .00 .30 .00 .30 .00 .30 .00 .30 .00 .30 .00 .30 .00 .30 .00 .30 .00 .30 .00 .30 .00 .30 .00 .30 .00 .30 .00 .30
8 8 9 9 10 10 11 11 12 12 13 13 14 14 15 15 16 16 17 17 18 18 19 19 20 20 21 21 22 22 23 23
Note the flow fluctuations with a balanced system (red line) caused by the changes in system pressures.
When ΔP controllers are installed, these fluctuations are not as evident, as the downstream pressure is
maintained relatively constant, irrespective of pressure fluctuations within the system.
Key results: 50% Load and 20% Flow:
Power usage balance system - 2,732 KWH/w
Power usage with ΔP control - 1,836 KWH/w
• Energy savings - 896 KWH/w (49%) !
Similar installation as the test, 2 way modulating
control valve in series with STAP ΔP controller
and STAD balancing valve.

6. Maintaining correct
pressure The TA Differential Range:
Above and below:
STAP & DA516 ΔP Controllers
STAP ΔP Controller in series with 2 way control valve installed on a AHU.
In this installation the authority of the control valve will be close to 1.
For the full engineering facts on balancing
and using ΔP control, get a FREE copy of our
handy book
Either download from our catalogue sizing
disc available through
info@tourandersson.com.au
or contact us directly (details below)
TA Hydronics has a complete solution for
the HVAC industry: balancing hardware,
engineering skills, system knowledge and
international team and project experience.
Tour & Andersson
Unit 25/148 Chesterville Road, Moorabbin 3189, Victoria,
Phone +61(3) 9553 3366 AUSTRALIA
Phone +61(3) 9553 3366, Fax +61(3) 9553 3733
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