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1
Chilled Water Distribution Systems
APPA Institute for
Facilities Management
New Orleans, LA
January 19, 2016
2
Purpose of Today’s Presentation
To provide a broad understanding of chilled
water distribution systems
Explore in some detail various distribution
system configurations
Provide some useful observations and
solutions
3
Agenda
System Concepts
– Definitions
– Basic Formulae
T
– Hydraulic Profile
System Components
System Configurations
4
WORDS OF WISDOM
It’s not how much you’ve got; it’s whether
you can use it.
Production Distribution Load
5
Definitions
System (Static/Fill) Pressure: The non-flowing pressure to which the system must be filled to assure flooding of the highest device.
– Static pressure is created by the weight of water in the system. Static pressure has no effect on pump capacity. If you consider a water piping system as being an upright loop of water confined in a pipe, the static pressure in one of the vertical pipes is caused by the weight of the water column in the pipe.
– Static Pressure is equal to .434 pounds per sq. inch per foot of water above the measurement gauge. For example, if the highest device is 20 feet above the gauge, the static pressure at the gauge will be: 20 x .434 which equals 8.6 psig. At various elevations above the gauge, the static pressure becomes correspondingly less. At 10 feet, it is 4.3 pounds per sq. in., and at the top, located 20 feet above the gauge, there is no pressure.
System pressure is usually set so that there is at least 5 psig measured at the highest device in the system.
QUESTION: What pressure must there be in the system if the highest device is located 120 feet above the chilled water makeup water inlet?
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ΔH= 120’
Fill Pressure, Makeup, and
Expansion
Makeup/Fill Water
ΔH
System Pressure = .434 psi/ft X 120’ + 5 = 57 psig
Makeup/Fill Water
ΔH= 120’
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Definitions (cont.) Dynamic Pressure:
– The flowing pressure the system pumps must develop to overcome the friction due to piping, coils, valves, fittings, and other devices in the system at a given flow rate.
– Head loss, measured in feet of head = 2.31 ft. W.C./psi (1/.434 psi/ft)
Design Pressure
– The dynamic pressure the system pumps must develop at the maximum flow in the system.
– The differential pressure between the supply and return piping at the pump, i.e. the total head
QUESTION: What will the supply and return pressures be in our 57 psig system if the design head loss at maximum flow is 100’ W.C.?
Supply Pressure = 100’ W.C. X .434 psig/ft + 57 psig = 100 psig
Return Pressure = 57 psig
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System Hydraulic Profile
Relative Distance from Plant
Plant Pumps
Supply Piping
Typical Bldg Load
Return Piping
Tota
l H
ead =
100’
57 psig
100 psig
Pre
ssure
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Basic Formulae
The heating and cooling capacity of water when it flows through a coil
(heating or cooling) can be calculated as follows:
Basic equation: Q = mcpΔT = cpVΔT
for water: Q = 60min/hr ·V · 8.33 lb/gal · 1.0 BTU/lb-oF · ∆T
= 500 x GPM x ∆T
Converting to refrigeration tons:
QTons = 500 x GPM x ∆T
12,000 BTU/Ton-hr
Q = heat rate (Btu/hr, kJ/hr)
m = mass flow (lbm//hr, kg/hr)
cp = specific heat @ const. press.
= density (lb/cu. ft.)
ΔT = temperature difference
between supply and return
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TGPMQtons
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Chilled Water System
Component Interactions
Pumps/ Piping
– Parallel Pumping
– Series Pumping
– Variable Speed Pumping
Effect of T on Pump Energy
Effect of T on Pump Flow
Effect of T on Dynamic Pressure
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Pumping Arrangements
1 Pump
2 Pumps
1 Pump
2 Pumps
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Varying Pump Speed
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TGPMQtons
13
ΔT vs. System HP For Fixed Load
400
148
62.5
32 18.5
11.7
7.8
5.5
4 3 2.3
1.8
0
100
200
300
400
500
4 6 8 10 12 14 16 18 20 22 24 26
Temperature Difference
Ho
rsep
ow
er
HP
Delta T vs. Req’d System HP
0
24
TGPMQtons
14
Specific Flow vs. ΔT
System Pump HP ~ Q3
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Dynamic Pressure vs T
• Increasing supply-to-return differential
temperature requires less flow for same heat
transferred
• Less flow in a given pipe system results in
lower velocity
• Lower velocity equals lower friction and lower
pressure loss
• Lower pressure and flow equals lower energy
Three Rules for Chilled Water System Optimization
Reduce Flow
Reduce Flow
Reduce Flow
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TGPMQtons
16
Chilled Water Distribution System
Configurations – Constant/Variable Flow Combinations
Primary
Primary/Secondary
Primary/Secondary/Tertiary
– Variable Direct Primary
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Constant Primary Only
(One unit on)
Load equals 1 chiller = 1000 gpm @ 12oF T = 500 Tons
Chiller
500 Tons
Control Valve Pump
1000 gpm
Bldg Coils
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Constant Primary Only
(Two units on) Control Valves
bypass excess
water into return
Load equals 1.2 chillers = 600 Tons = 2000 gpm @ 7.2oF T
Chiller
500 Tons
Pump
1000 gpm
Chiller
500 Tons
Pump
1000 gpm
Bldg Coils
CV
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Constant Primary / Secondary
Building Secondary Pumps
Chiller
500 Tons
Pump
1000 gpm
Chiller
500 Tons
Pump
1000 gpm
Bldg Coils “Bridge”
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Constant Primary / Secondary / Tertiary
Building Secondary Pumps
Chiller
500 Tons
Pump
1000 gpm
Chiller
500 Tons
Pump
1000 gpm
Bldg Coils “Bridge”
Secondary Pump
21
Constant Primary / Variable Secondary
(primary and secondary pumps in central plant )
Control Valve
Chiller
500 Tons
Chiller Pump
1000 gpm
Chiller
500 Tons
Chiller Pump
1000 gpm
Bldg Coils
Variable Secondary Pump
3000 gpm max.
Bypass
(Bridge)
System flow less than chiller flow System flow more than chiller flow
Chiller staging indicated by flow direction in the bridge
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Variable Primary Only
(One unit on)
Load equals 1 chiller = 1000 gpm @ 12oF T = 500 Tons
Chiller
500 Tons
Control Valve
Bldg Coils
VF Pump
1000 gpm
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Variable Primary Only
(Two units on) Control Valves
close against
increased pressure
Load equals 1.2 chillers = 600 Tons = 1200 gpm @ 12oF T
Chiller
500 Tons
VF Pump
600 gpm
Chiller
500 Tons
VF Pump
600 gpm
Bldg Coils
Chiller and flow staging accomplished by measurement of
P between supply and return at selected location
QUESTION: How can we improve this scheme?
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Questions & Answers Thank You!
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