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Engineering correct heating and cooling balance in
buildings
Bulletin 1005-1
Hydronic System Design Update
Controlling Differential Pressure is the secret to perfect room temperature in variable flow systems
TechTALK #4
So why is Differential Pressure Control essential in most variable flow systems?
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.
The modulating 2 way control valve creates a supplementary pressure drop in the hydraulic circuit to limit the water flow to the required value. The water flow depends on the differential pressure applied to the valve.
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 then being applied to the only moving component: the 2 way modulating control valve.
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.
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.
In this paper, we will discuss the effects of varying flows and pressure on the performance of the system & equipment within the system.
Pump head x Flow
Pump EfficiencyPumping Costs C0 +
Above: The differential pressure applied to the control valve depends on its degree of opening
TerminalControl Valve
Balancing Valve
The Need for Differential Pressure Control
Climate conditions affect plant loadings
SimulationBalanced System • 100% Load and Flow
Australian conditions
75% (varies slightly by state) of the cooling season in Australia plant loading requirements will be 50% or less.
Load variations are heavily influenced by:
• Sunshine effects (up to 750 W/m² for a East façade in December around 4pm at 50° South).
• Building occupancy (1 sitting person: ±110 W, computers).
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.
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?
% of cooling season below the load
0 10 20 30 40 50 60 70 80 90
100 Average cooling load [%]
0 10 20 30 40 50 60 70 80 90 100
0
5
10
15
20
25
30
Jan Feb
March
April
May Jun
eJul
yAu
gust Sep Oct Nov Dec
Seasonal VariationAverage Monthly Temperatures (degree C)
Figures supplied by the Melbourne Bureau of Meteorology covering the year 2006.
Computer simulation using TA-Simul
Control Valve Authority is defined as:
Control Valve Authority b = ΔP across control valve at design flow ΔP valve closed
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°C
0
10
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0 10 20 30 40 50 60 70 80 90 100
Water Flow %
Valve Lift %
0.43
0.18
Differential Pressure variations due to Load & Flow
Balanced System • 50% Load, 20 % Flow
• Cooling Systems operate at 50% load or less for around 75% of the cooling season!
Control valve authority: at design flow and 20% of design flow
100% Load & FlowAuthority b = 0.43
50% Load, 20% FlowAuthority b = 0.18
(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 of 0.07 L/sec. The ΔP across the control valve has increased from the design value 45.8 kPa to a massive 246.7 kPa!
This affects the control valve authority, and performance from 0.43 to 0.18 - this control valve is now in ON/OFF mode (refer to graph).
20 %
flow
0%
20%
40%
60%
80%
100%
0% 20% 40% 60% 80% 100%0%
20%
40%
60%
80%
100%
120%
0% 50% 100% 150% 200%
50 %
load
4% press.
drop
Pressure drops reduced to 4%of their initial value
The pump head applies itself almostentirely on the 2-way automatictemperature control valves
Emission
Flow Flow
ΔP piping
At constant supplywater temperature
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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)
0
10
20
30
40
50
60
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100
0 10 20 30 40 50 60 70 80 90 100
Water Flow %
Valve Lift h%
0.43
0.18
0.47
100% Load & FlowAuthority b = 0.43Balanced System
50% Load, 20% FlowAuthority b = 0.18Balanced System
50% Load, 20% FlowWith ΔP controlAuthority b = 0.47
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.
Energy Savings & Improved Performance
ΔP control assists in achieving Green Star Ratings
Case study: Asian University
Tests were carried out on STAP ΔP controller and STAD balancing valves servicing an AHU at the University, logging the measured flow, return water temperature and power consumption.
Before each test, the system was balanced at design flow, the STAP ΔP controller was tuned to maximise energy usage.
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.
Test conditions: 50% Load and 20% Flow.
The first test was carried out with the capilliary pipe to the STAP ΔP controller disconnected, disabling the ΔP controller from the circuit. Continuous logging of measurements were taken at 30 minute intervals for 7 days.
Tests were repeated over the same period in time with the ΔP controller connected, continuously logging the flow, return water temperature and power consumption.
The results in the graph shown here were remarkable:
0
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0.4
0.6
0.8
1.0
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2.0
8.008.30
9.009.30
10.0010.30
11.0011.30
12.0012.30
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20.0020.30
21.0021.30
22.0022.30
23.0023.30
Without ΔP control With ΔP controlFlow Rate L/sec
Time
Graph drawn from actual measured data over one day during the summer months
20% of design Flow
Maintaining correct pressure
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.
Tour & Andersson
Unit 25/148 Chesterville Road, Moorabbin 3189, Victoria,
AUSTRALIA
Phone +61(3) 9553 3366, Fax +61(3) 9553 3733
an Indoor Climate business of IMI plc
For the full engineering facts on balancing and using ΔP control, get a FREE copy of our handy book
Phone +61(3) 9553 3366www.tourandersson.com.au
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