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Chilled Water Distribution Systems

APPA Institute for

Facilities Management

New Orleans, LA

January 19, 2016

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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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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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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

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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

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TGPMQtons

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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

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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)

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

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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!