water-side systems _ system design -- bbse3006_1011_05

8/12/2019 Water-Side Systems _ System Design -- Bbse3006_1011_05

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BBSE3006: Air Conditionin and Refri eration IIhttp://www.hku.hk/bse/bbse3006/

Water-side Systems: System Design

Dr. Sam C M Hui

The University of Hong Kong

- .Jan 2008


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Contents

System Components

Heat Transfer Calculations

P p ng System Des gn


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Chilled

water

system

Condensingwater system

(Source:Fundamentals of Water System Design)

Water systems in HVAC


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Basic Concepts

Water s stem desi n needs evaluation of

Space loads

Occu anc atterns

Indoor environmental requirements

Transmission, solar radiation, infiltration, ventilation air,, , ,

Effective system design must consider

u - oa an part- oa con t ons

Occupants comfort conditions


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Source Load


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Source Distribution Load


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THREE-WAY

CONTROL VALVE

TWO-WAY

CONTROL VALVE


Source Distribution Part-Load


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Basic Concepts

T es of water s stem

Closed system

Open system

, . .

cooling towers

Flow cannot be provided by static head differences

Pumps do not provide static lift

The entire piping system is always filled with water


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DistributionPump


Chilled water system (closed system)


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Cooling tower (open system)


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Basic Concepts

HVAC water s stems can be classified b

Operating temperature

Pressurization Piping arrangement

Pumping arrangement

Piping materials

Hot water: black steel, hard copper

Condenser water: black steel, galvanized ductile iron, PVC


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Basic Concepts

O en water s stems e. . usin coolin tower

Closed water systems

e water system - o , a

Condenser water (CW) system Dual temperature water system

Low tem . water LTW s stem Max. 120 oC < 1100 kPa

Medium temp. water (MTW) system [120-125 oC, < 1100

High temp. water (HTW) system [> 175 oC, > 2070 kPa]

nce- roug sys em, e.g. sea wa er sys em


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Multi le chiller variable flow chilled water s stem(Source:ASHRAE HVAC Systems and Equipment Handbook 2004)


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Direct return and reverse return


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Chilled water system direct return with balancing valves


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Dual-temperature, four-pipe water system


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Condenser cooling tower system


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System Components

Basic com onents of water h dronic s stem

Source system (chiller or boiler)

Pump system Distribution system

Expansion chambers


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Basic com onents of water h dronic s stem(Source:ASHRAE HVAC Systems and Equipment Handbook 2004)


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System Components

Source

It is the point where heat is removed from a cooling system

Source efficiency as a function of load

ommon source ev ces

Cooling source: electric chiller, absorption chiller, heat

pump evaporator, water-to-water heat exchanger Heatin source: hot water enerator or boiler, steam-to-

water heat exchanger, water-to-water heat exchanger, solar

collector panels, heat pump condenser, heat recovery device


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System Components

Two main considerations in selectin a source device

Design capacity

-

Turndown ratio = (min. capacity / design capacity) x100%

=

Use of multiple chillers/boilers

To achieve better operation efficiency

Facilitate maintenance and standby backup


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Multiple chiller example (2 chillers)


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Multiple chiller example (3 chillers)


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System Components Desi n trade-offs

Improved efficiency vs initial installation cost

Must design temperatures and temperature ranges by

components

For exam le if conditioned s ace at 25 C 50% RH has

dewpoint temperature 13 C, then max return watertemperature should be near 13 C. Lowest practicaltemperature for refrigeration, considering freezing andeconomies, is about 4.5 C. Thus, chilled water systems are

se a . - .


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System temperatures


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System Components

Load s stems are devices terminal units that conve

heat to the water for cooling or from the water for

Most of them are water-to-air finned coil heat exchangers or

wa er- o-wa er ea exc angers

Cooling load devices, e.g.

Cooling coils in air-handling units (AHUs), fan coil units , . .

Heating and preheat coil in AHUs


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Single-zone central AHU (cooling and heating coils)


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Fan coil unit


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Sensible coolin or heatin of air

q = Qaa cpt

, . ,

kJ/kg.K, thus, q = 1.2 Qat

ater co or eat exc anger

q = UA (LMTD)

LMTD = log mean temperature difference = t -t / ln t / t

Depends on surface area, overall heat transfer coefficient,

eometr of heat transfer surfaces etc.


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= .

= 1.2 (2.5 m3/s) (55C 15C) = 120 kW


Sensible heating example


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Assume the coil has a U of 850 W/m .C/row.

The coil has four rows.


Coil LMTD example


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Water and air temperatures across the coil


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First, determine LMTD:

LMTD = (tmax -tmin) / ln (tmax/ tmin)

tmax = 60 15 = 45 tmin = 70 55 = 15

Thus, LMTD = (45 15)/ ln (45/15) = 27.3 C

Using LMTD, find q: . . .

= 100,246 W = 100.25 kW


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Latent coolin and dehumidification of air

Both sensible and latent heat transfer

total

W = mass flow rate of cooled medium, kg/s

cooled medium, kJ/kg

= =total a a , total . a


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q = 1.2 Qa h

= 1.2 2.5 m3/s 54.5 32C = 67.5 kW


Cooling/dehumidification coil example


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Heat Transfer Calculations Heat transferred to or from water

qw = m cpt (kW)

en vo ume ow ra e s s use

qw = 0.001 w cp Qwt As w = 1000 kg/m3, cp = 4.19 kJ/kg.K,

Therefore, qw = 4.19 Qwt

For design or diagnosis of a system, we often assume


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Piping System Design Piping system is a key component of the distribution system

Must consider 3 important steps: Establish the i in desi n hiloso h & ob ectives

Size the pipes

Calculate or determine the ressure dro s

Relationship between pressure and head

= z where = ressure Pa N/m2 z = head m

Pressure drop

[ g Z1 + V12/2g + p1] = [ g Z2 + V2

2/2g + p2] + p


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Bernoullis Theorem


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Bernoulli piping example


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Piping System Design

Bernoulli i in exam le:

According to the Bernoulli Equation Z + V 2/2 + = Z + V 2/2 + +

Thus, p = g(Z1 - Z2) + /2g (V12 - V2

2) + 103 (p1 - p2)

V1 = V2 Z = 0

p = 998.97 x 9.81 (-30) + 0 + 103 (700 - 500) = 206,000 Pa = 206kPa

A total loss of 206 kPa due to piping and fittingfriction and elevation head loss

For cold water, 1 m static head is about 9.8 kPa


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Direct return s stem

Length of supply and return piping through

May cause unbalanced flow & require carefula anc ng

Reverse return s stem

Provide equal total lengths for all terminal circuits

A combination of direct and reverse systems


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Direct return piping


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Direct return pressure drop diagram


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Reverse return piping


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Reverse return pressure drop diagram


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Direct return riser and reverse zone piping


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Other considerations

Is the system to be constant flow?

Is variable flow being considered?

Will the pump speeds be varied with the load?

How to put and design control valves?

How to use balancing valves?


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Sizing Piping

Principles Based on friction loss per running meter of pipe

Fluid velocity as a limiting selection parameter

-

VLVL

22

gDD 2

2

f= friction factor

L = pipe length, m

= p pe ame er, m

V= fluid average velocity, m/s

= densit of fluid k /m3

g= gravitational constant, m/s2


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Experimental arrangement - determining head loss in pipe

Si i Pi i


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Sizing Piping

General design criteria

Pipe friction loss = 400 to 500 Pa/m For controlling velocity noise, velocity limit = 2.5 m/s

Need to know the fluid mechanics theories if accurate pipesizing or analysis is needed

eyno s num er e =

Two different conditions:

=

Turbulent flow (Re > 2000)

In laminar flow range, the friction factor,f= 64 / Re

Pipe roughness factor (), relative roughness (/D)

Use of Moody Chart to show the relationship between friction

ac ors an e


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Reynolds number, friction flow and relative roughness


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Moody chart - friction factors and Reynolds numbers

Si i Pi i


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Sizing Piping

ASHRAE Fundamentals Handbook refers to

Colebrook Equation for determining the friction factor

7.181

fDf Re

.

-

alternative to Darcy-Weisbach Equation8522.1

6103.35

Q

. Cd

Si i Pi i


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Sizing Piping

Darc -Weisbach E uation Colebrook E uation and

Hazen-Williams Equation are fundamental to

piping

For practical design, charts and tables calculated from

these equations are developed for typical pipes (e.g.

medium steel, copper and PVC pipes)


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Pressure loss 20C water in medium steel pipe


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(Source: ASHRAE Handbook Fundamentals 2005, Chp. 36)

Sizing Piping


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Sizing Piping

Valve and fittin losses

May be greater than pipe friction alone

KhKp LL

2

or

2

KL = loss coefficient (Kfactor) of pipe fittings

Geometr and size de endent

May be expressed as equivalent lengths of straight pipe v

Volume flow rate /pAQ v


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(Source: Larock, Jeppson and Watters, 2000:Hydraulics of Pipeline Systems)


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water-side systems _ system design -- bbse3006_1011_05

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