wsd_chp_7

© 2007 ASHRAE Hong Kong Chapter Slide 1

Fundamentals of Water System Design

17, 18, 24, 25 January 2007

ASHRAE Hong Kong Chapter Technical Workshop


Chapter 7:Piping System Development

•Piping System design•Direct Return Analysis•Reverse Return Analysis•Primary-Secondary Analysis

•Type of Pumps and Valves•Primary-Secondary Application Study•Antifreeze Solutions for Low Temperature Application•Pumping Design Factors


Piping System Design


Piping System Design Flowchart


Typical Building Layout

Building usage•Hotel •Shopping Mall•Services apartment•Office•Mechanical floor


Determine Loads andConsult References


Determine the system to be used and develop a concept for part-load control

SystemFCU + PAUVAV systemAHU (all air system)others

PlantroomWater cooled chillersAir cooled chillersSplit system

ControlApplicationOperation hourZoneIAQ/Energy consideration

O&MMaintenance and Services


Develop Piping/PumpingSystem Concept

Chiller combination• Full load/Part load/• Emergency• Standby

Pumping system• Primary –Secondary• Variable Primary sys

Water distribution system• Direct return• Reverse return• Direct + reverse return


What is Direct Return and Reverse Return?

Coil Coil Coil Coil Coil


What is Direct Return and Reverse Return?

CoilCoil Coil Coil Coil

CoilCoil Coil Coil Coil

Direct Return

Reverse Return


Load System Example


Piping Detail at Load Coil


Friction Loss, Medium Steel Pipe


Direct Return Piping Layout


Direct Return System

Flow

Unit 3

Unit 1

Unit 2

Unit 4

A

BB'

CC'

D D'

EE'

F

Piping pressure drop

Unit 1 80 kPaUnit 2 98 kPaUnit 3 108 kPaUnit 4 90 kPa



Flow

Unit 3

Unit 1

Unit 2

Unit 4

A

BB'

CC'

D D'

EE'

F

Piping pressure drop and flow

Unit 1 80 kPa 5.8 L/sUnit 2 98 kPa 5.2 L/sUnit 3 108 kPa 5 L/sUnit 4 90 kPa 5.5 L/s

Design flow: 5L/s

Assume coil pressure drop: 30kPa



Flow

Unit 3

Unit 1

Unit 2

Unit 4

A

BB'

CC'

D D'

EE'

F

Design flow: 5L/s



Unit 1 65 kPa 6 L/sUnit 2 83 kPa 5.3 L/sUnit 3 93 kPa 5 L/sUnit 4 75 kPa 5.6 L/s


Reverse Return Piping Layout


Reverse Return System

Unit 4Unit 1 Unit 2 Unit 3

A B C D E

F

B' C' D' E'


Unit 1 139 kPa 5.36 L/sUnit 2 160 kPa 5 L/sUnit 3 160 kPa 5 L/sUnit 4 139 kPa 5.36 L/s

Total 20.72 L/s

Design flow: 5L/s



Reverse Return System

Unit 4Unit 1 Unit 2 Unit 3

A B C D E

F

B' C' D' E'

Design flow: 5L/s


Unit 1 124 kPa 5.41 L/sUnit 2 145 kPa 5 L/sUnit 3 145 kPa 5 L/sUnit 4 124 kPa 5.41 L/s

Total 20.82 L/sAssume coil pressure drop: 15kPa


Comparison

Coil Pressure drop = 30kPa Coil Pressure drop = 30kPa

flow flowUnit 1 80 kPa 5.8 L/s Unit 1 139 kPa 5.36 L/sUnit 2 98 kPa 5.2 L/s Unit 2 160 kPa 5 L/sUnit 3 108 kPa 5 L/s Unit 3 160 kPa 5 L/sUnit 4 90 kPa 5.5 L/s Unit 4 139 kPa 5.36 L/s

Total 21.5 L/s Total 20.72 L/sPump kw = 2.3 kw Pump kw = 3.3 kw

Coil Pressure drop = 15kPa Coil Pressure drop = 15kPa

flow flowUnit 1 65 kPa 6 L/s Unit 1 124 kPa 5.41 L/sUnit 2 83 kPa 5.3 L/s Unit 2 145 kPa 5 L/sUnit 3 93 kPa 5 L/s Unit 3 145 kPa 5 L/sUnit 4 75 kPa 5.6 L/s Unit 4 124 kPa 5.41 L/s

Total 21.9 L/s Total 20.82 L/s

Pump kw = 2.1 kw Pump kw =3.1 kw

Direct Return System Reverse Return System


Piping Detail: Load Coil Primary-Secondary


Primary-Secondary Piping


Primary/Secondary system

Coil Pressure drop = 30kPa

flowUnit 1 47.3 kPa 8 L/sUnit 2 65.3 kPa 5.8 L/sUnit 3 75.3 kPa 5 L/sUnit 4 57.3 kPa 6.6 L/s

Total 25.4 L/sPrimary Pump kw = 1.9 kwSecondary Pump kw = 4 x 0.2 = 0.8 kw

Total Pump kw = 2.7 kw

Compare to direct return system, pump kw = 2.1 kw


Purpose of Pumping Systems

Transport sufficient water through the piping system

at the minimum differential pressurethat will satisfy all connected loads at

different load conditions


Why balanced flow is important?

Drawback of unbalanced system: Cannot meet the design flow and capacity at the air terminal unit Waste energyShort circuit (hydronic)Chiller hunting


How to balance the system?

Add Balancing DeviceFixed orificeManual balancing valvesConstant flow valve Pressure Independent control valve

Remember

Flow rate Q = Cv ‧A ‧√ΔP


Method 1 & 2The manual balancing valve

(Similar as orifice)

Is an adjustable orifice - not a flow controller.Must be manually adjusted according to pressure differential.Introduces manual error into system performance.


The manual balancing valve adjustment

Requires special equipment and training on procedure.Have to access valves on-site in ceilings etc. Commissioning after installation, system filling, & pump commissioning.Requires time for commissioning.Difficult to re-balance if the project completion will be in staged or modified


The manual balancing act

As a manual valve is adjusted, it not only changes the coil flow, it changes the total flow in the common pipe. The pressure differentials and flows across parallel circuits are upset and then must be re-adjusted.

- +- + + +

Valve throttled & flow reduced

Common pipe flow &

pressure loss will be reduced

Pressure differential & flow across the valve increased

1

It will be a static system, cannot response to a dynamic or variable flow system


BranchesRisers

Branch and risers in Manual balancing system

Don’t forget those additional regulating valves:


Method 1 & 2The manual balancing valve

(Similar as orifice)

After commissioning or adjustment of the manual balancing valves, the Cv and A (area) of each valve is fixed.The flow rate will then be pressure dependent.



Method 3Constant Flow valve

CharacteristicBelow the control range the cartridge is a fixed orifice & flow can be varied by a 2 way control valve In the dP control range, flow is limited to design +/-5%

Flow

Differential Pressure (dP)

Range Minimum

kPaD

Design Maximum

Range Maximum

kPaD

00


By using constant flow valve

Either in 2-way or 3-way control valves system

PROLess valves (not required in branch and riser)Reduce T&Cwork and time

CONFlow modulating depend on control valves in part loadNot fully dynamic balancing system


Method 3Constant Flow valve

After installation of the constant flow valves, the (Cv ‧A)of each valve will compensate the variation of the (√ΔP).The flow rate will then be kept constant.

However, it is suitable for the constant flow application like,Constant flow chillers and pumpsMost of the FCU application (constant flow)



Method 4Pressure Independent Control Valve

Pressure independent control valveFunction:-1. System pressure independent2. Flow rate Modulating control3. Pre-set maximum flow


Pressure Independent Control Valve Characteristic

Pre-set Maximum flow for each AHUFlow rate varies

according to the temp controller or DDC input

signal (2-10V or 4-20mA)

valve will then hold the flow rate constant regardless of the change in pressure differential.


System using Pressure Independent Control valve

CharacteristicLess valves, combine the control function.Elimination of branch balancing valves & reverse return pipe work.Valve authority = 100%Pre-set the max flow of each AHU and save lot of time in commissioning work.No need to re-balance the system even the project is staged or modified.


Method 4Pressure Independent Control Valve

After installation of the pressure independent control valves, the (Cv ‧A) of each valve will compensate the variation of the (√ΔP) at various load at any time.The flow rate will then be pressure independent, only temperature / load dependent

It is suitable for most of the modulating control applications,AHUsPrecise flow control FCUs

Flow rate Q 1% Q 100% = (Cv ‧A) 1-100‧√ΔP


The pumping system will be required to operate under various load conditionsVariable flow system differential pressures throughout the system will be dynamic.Hydronic systems should be hydraulically modeled to design for full load and part load performance

What happen when the system is in part load?


System at Part load

Coil Coil Coil Coil Coil

OFF

60%load

83%load

31%load

100%load

★Flow rate required for each AHU or branch is varying all the time


Valve must be perfectly sized to provide exact resistance for pressure differential when fully open to provide design flow

What happen in typical Control valve for part load condition


Typical Control Valve is Pressure Dependence

Standard 2 way valves vary opening area only – but not flow.As pressure differential varies, the flow varies.

Q = dP * Orifice constant

Typical Control valve for part load


Once below design flow, as each 2-way control valve reduces flow, it increases the pressure differential and flows across parallel circuits.

T

Time

System pressure varies affect the flowrate passing through valves and coilsThe temperature is then altered due to the pressure fluctuation.

Control valve for part load


A system using typical control valve and manual balancing valve

VFD Coil

Coil

DP

6.3 L/s35 kPa

6.3 L/s35 kPa

35 kPa192 kPa

35 kPa12 kPa

82 kPa262 kPa310 kPa

PUMP Coil #1 REMOTE LOAD0

480PR

ESSU

RE

kPa

Typical control valve

Manual balancing valve


A system using typical control valve and manual balancing valve

PRESSURE DROP OF CONTROL VALVE AND MANUAL BALANCING VALVE

AT VARIOUS LOAD CONDITIONSFull Flow

75% Flow

50% Flow

25% Flow

10% Flow

Branch Flow (L/s) 6.3 4.7 3.2 1.6 0.6

Branch ∆P (kPa) 262 262 262 262 262

Coil ∆P (kPa) 35 19 9 2 0.7Manual, Balancing Valve ∆P (kPa)

192 109 48 12 2

Control Valve ∆P (kPa) 35 134 205 248 259

As ∆P across typical control valve increase

seriously during part load

Waste energy


Balancing Considerationsin Variable Flow Systems

• Too large a balancing valve pressure drop will affect the performance and flow characteristic of the control valve.• ASHRAE 2003 Applications Handbook, page 37.8


Options to Consider• No manual balancing valves at coils• Automatic differential pressure control to reduce

differential pressure• Pressure-independent control valves (with Flowrate

pre-set function)

Options NOT to Consider:• Balancing valves for variable speed pumps

Balancing Considerationsin Variable Flow Systems


Four-Zone Heating System


Typical Building Layout

Building usage•Hotel •Shopping Mall•Services apartment•Office•Mechanical floor


Primary-Secondary Pumping, Four-Zone Heating System


Primary-Secondary Bridge Energy


Primary-Secondary Pumping, Four-Zone Heating System


Coil With Glycol Heat Exchanger and Pump


Pumped Coil WithFace-Bypass Dampers


Specific Heats ofAqueous Ethylene Glycol


Specific Heats ofAqueous Propylene Glycol


One-Shot Chemical Feeder


Thank you

Edward Tsui

Email: [email protected]

wsd_chp_7

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