Chilled Water Distribution Systems
APPA Institute for Facilities Management February 6, 2014
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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
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Agenda System Concepts – Definitions – Basic Formulae ∆T
– Hydraulic Profile
System Components System Configurations
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APPA Institute - Dallas, TX Feb 2014
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WORDS OF WISDOM It’s not how much you’ve got; it’s whether you can use it.
Production
Distribution
Load
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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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Fill Pressure, Makeup, and Expansion
∆H= ∆H120’ ∆H= 120’ Makeup/Fill Water
Makeup/Fill Water
System Pressure = .434 psi/ft X 120’ + 5 = 57 psig 6
APPA Institute - Dallas, TX Feb 2014
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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 7
System Hydraulic Profile
Pressure
Total Head = 100’
100 psig
Typical Bldg Load Plant Pumps Supply Piping
57 psig
Return Piping
Relative Distance from Plant
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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 = mcp∆T = ρcpV∆T for water: Q = 60min/hr ·V · 8.33 lb/gal · 1.0 BTU/lb-oF · ∆T = 500 x GPM x ∆T Q = heat rate (Btu/hr, kJ/hr) m = mass flow (lbm//hr, kg/hr) Converting to refrigeration tons: cp = specific heat @ const. press. QTons = 500 x GPM x ∆T ρ = density (lb/cu. ft.) 12,000 BTU/Ton-hr ∆T = temperature difference
Qtons =
GPM × ∆T 24
between supply and return
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APPA Institute - Dallas, TX Feb 2014
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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 10
Pumping Arrangements
2 Pumps
1 Pump
2 Pumps
1 Pump
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Varying Pump Speed Qtons
GPM × ∆T = 24
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APPA Institute - Dallas, TX Feb 2014
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Delta T vs. Req’d System HP Qtons =
GPM × ∆T 24
∆T vs. System HP For Fixed Load
400 300 200
2. 3
1. 8
4
3
7. 8
5. 5
18 .5
11 .7
62 .5 32
100 0
HP
14 8
Horsepower
40 0
500
0
4
6
8 10 12 14 16 18 20 22 24 26 Temperature Difference 13
Specific Flow vs. ∆T
System Pump HP ~ Q3
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Dynamic Pressure vs ∆T Qtons =
GPM × ∆T 24
• 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 15
APPA Institute - Dallas, TX Feb 2014
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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) Pump 1000 gpm
Control Valve
Chiller 500 Tons
Bldg Coils
Load equals 1 chiller = 1000 gpm @ 12oF ∆T = 500 Tons 17
CV
Constant Primary Only (Two units on) Pump 1000 gpm
Control Valves bypass excess water into return
Chiller 500 Tons
Bldg Coils Pump 1000 gpm
Chiller 500 Tons
Load equals 1.2 chillers = 600 Tons = 2000 gpm @ 7.2oF ∆T 18
APPA Institute - Dallas, TX Feb 2014
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Constant Primary / Secondary Pump 1000 gpm
Chiller 500 Tons
Building Secondary Pumps
Bldg Coils
“Bridge” Pump 1000 gpm
Chiller 500 Tons
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Constant Primary / Secondary / Tertiary Pump 1000 gpm
Chiller 500 Tons
Building Secondary Pumps Secondary Pump
“Bridge”
Bldg Coils
Pump 1000 gpm
Chiller 500 Tons
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Constant Primary / Variable Secondary (primary and secondary pumps in central plant ) Chiller Pump 1000 gpm
Chiller 500 Tons
Variable Secondary Pump 3000 gpm max.
Bypass (Bridge)
Control Valve
Bldg Coils
Chiller Pump 1000 gpm
Chiller 500 Tons
less than System flow more thanchiller chillerflow flow
Chiller staging indicated by flow direction in the bridge 21
APPA Institute - Dallas, TX Feb 2014
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Variable Primary Only (One unit on) VF Pump 1000 gpm
Control Valve
Chiller 500 Tons
Bldg Coils
Load equals 1 chiller = 1000 gpm @ 12oF ∆T = 500 Tons 22
Variable Primary Only (Two units on) QUESTION: How can we improve this scheme? VF Pump 600 gpm
Control Valves close against increased pressure
Chiller 500 Tons
Bldg Coils
VF Pump 600 gpm Chiller 500 Tons
Chiller and flow staging accomplished by measurement of ∆P between supply and return at selected location
Load equals 1.2 chillers = 600 Tons = 1200 gpm @ 12oF ∆T 23
Questions & Answers Thank You!
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APPA Institute - Dallas, TX Feb 2014
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