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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:Fundamentals of Water System Design)
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
(Source:Fundamentals of Water System Design)
Chilled water system (closed system)
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(Source:Fundamentals of Water System Design)
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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(Source:Fundamentals of Water System Design)
Direct return and reverse return
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(Source:Fundamentals of Water System Design)
Chilled water system direct return with balancing valves
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(Source:Fundamentals of Water System Design)
Dual-temperature, four-pipe water system
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(Source:Fundamentals of Water System Design)
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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(Source:Fundamentals of Water System Design)
Multiple chiller example (2 chillers)
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(Source:Fundamentals of Water System Design)
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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(Source:Fundamentals of Water System Design)
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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(Source:Fundamentals of Water System Design)
Single-zone central AHU (cooling and heating coils)
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(Source:Fundamentals of Water System Design)
Fan coil unit
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Heat Transfer Calculations
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
(Source:Fundamentals of Water System Design)
Sensible heating example
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Assume the coil has a U of 850 W/m .C/row.
The coil has four rows.
(Source:Fundamentals of Water System Design)
Coil LMTD example
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(Source:Fundamentals of Water System Design)
Water and air temperatures across the coil
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Heat Transfer Calculations
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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Heat Transfer Calculations
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
(Source:Fundamentals of Water System Design)
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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(Source:Fundamentals of Water System Design)
Bernoullis Theorem
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(Source:Fundamentals of Water System Design)
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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Piping System Design
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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(Source:Fundamentals of Water System Design)
Direct return piping
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(Source:Fundamentals of Water System Design)
Direct return pressure drop diagram
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(Source:Fundamentals of Water System Design)
Reverse return piping
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(Source:Fundamentals of Water System Design)
Reverse return pressure drop diagram
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(Source:Fundamentals of Water System Design)
Direct return riser and reverse zone piping
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Piping System Design
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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(Source:Fundamentals of Water System Design)
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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(Source:Fundamentals of Water System Design)
Reynolds number, friction flow and relative roughness
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(Source:Fundamentals of Water System Design)
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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(Source:Fundamentals of Water System Design)
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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