Lecture 3 PDA 2011

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Process Design and Analysis

Lecture 3

Junping Cai MSc E.E, PhD

Office: Block E, Center for Product Development (CPD) Email: [email protected]

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Refrigeration system components

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RECAP FROM LAST TIME

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Summary - lessons learned lecture 2

• More thermodynamics

– Conservation of energy for CM processes from state 1 to 2

– Conservation of energy for steady CV processes

– Calculations with u, h, and c for ideal gasses, solids, and liquids

– Solids and liquids

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• More thermodynamics

– Explain what a reversible process is and why real processes are irreversible

– Reversible process has no losses or friction

– Reversible and adiabatic (Q=0 J) process is an isentropic process (ds= 0 J/kg-K).

– Irreversible process (sliding contact friction, unrestrained expansion, viscous fluid friction, heat transfer at final ΔT)

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• 1-stage vapour compression cycle, continued

– Differences between ideal and real 1-stage vapour compression cycle.

– Understand how TE and TC impacts the energy efficiency

– Define and calculate COP (“what we get / what we pay”)

– Learn about simple component models for the expansion valve, compressor and heat exchangers (UA value).

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Real refrigeration cycle

Sub-cooling, superheat, pressure losses, and frictions losses contributed to a less efficient cycle

Rules of thumb: COP improves 2-4% when per 1°C TE is raised or TC lowered

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COP – Coefficient Of Performance

2w

Evaporation temperature TE

4

3

Condensation temperature TC

2s

1

Dh = real compressor work [kJ/kg]

2

Dh = cooling capacity [kJ/kg]

1 4 1 4 1 4

2 1 2 12 1

e

is

w sw

m h hQ h h h hWhat you getCOP

What you pay W h h h hm h h

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Evaporator and Condenser

Refrigerant

Ambient

ambT

eT

e amb eQ U A T T

U: Overall heat transfer coefficient A: Surface area

Dimensioning - example:

0 . 10 . 10amb e eT C T C Q kW 10

110

U A kW K

Find evaporator with UA-value of 1 kW/K at dimensioning temperatures, with selected refrigerant

c c ambQ U A T T

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UA-Value

Tamb=20°C

Tcold=0°C

Small UAevaporator Large UAevaporator

Small UAcondenser Large UAcondenser

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Thermostatic expansion valve Operating principle

Evaporator Te

Exit temperature T1>Te

Vessel (bulb) with two-phase refrigerant at temperature T1

Te

T1

p1

pe

pe

Expansion Valve

p1

DTsh = p1-pe = “spring”

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Compressor sizing

BoreBore

Stroke

Displacement rate: Max theoretical volume flow through compressor :

2

SpeedMoved volume per revolution

4d revV Bore Stroke n

Real volume flow = volume flow in suction line = 1V

1V

Volumetric efficiency: 1

v

d

V

V

Sizing: Find from and dV 1V v

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• Direct and indirect systems (secondary systems)

– Keep refrigerant away from production areas

– Minimize refrigerant charge

• Refrigerants

– Properties (performance , p & T, safety, compatibility, cost etc.)

– ODP, GWP, TEWI, indirect/direct GHG emissions

– Energy efficiency important for the indirect emissions

– Common types and R-designation

• Secondary refrigerants (see “Stoecker”)

– Properties for secondary refrigerants

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Direct and indirect system

Chilled water system

Load Production Heat rejectionDistributionDistribution

Condenser water systemChiller

Chiller 3

M

Constant pumps120/240/240 m3/hrs

dP = 6+2+2 = 10 mVS

TC min. = 10°C

Control fouling risk

Back flush / CIP

M

M

2 x Cooling tower2 x 1.75 MW (30/35°C)

Remote sump

VSD fan (TC_opt)

Pump constant

dP= 6 mVS

Chiller 2

Chiller 1

Variable prim. pumps3 x 170 m3/hrs, 1 x standby

Flow (dP) across 2-way valve

Leq = 500 m, DN300 or 2xDN200

dP_mains = 5 mVS

dP_pump = 6+5+6+2 = mVS

By-pass for min. flow

AHU40 X 75 kW, 28.500 m3/hrs, SHR=0.9

Hand reg. valve + 2-way reg. valve

Leq = 50 m, DN65

dP = 2+2+2 = 6mVS

Load 40% base + 60% variable

EvaporatordP = 6 mVS

Flow min. 70%

Flow (dP) across 2-way valve

Compressor max. 4 starts/hour

Capacity 20+40+40%

TTFS

TTFS

TT

TT TT

TT

TTFS

TT RH

TT

TT

Indirect system: Circuit with secondary refrigerant between point of use and the primary refrigerant

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HCFC • R22

CFC: • R12, R502

Refrigerants

“Natural”

Ammonia (R717)

Water (R718)

Hydrocarbons • Propane (R290) • Propylene (R1270) • Isobutane (R600a)

Carbon dioxide (R744)

Air (R729)

“Synthetic”

HFC • R134a • R404A / R507 • R410A • HFO-1234yf (R-1234yf )

Refrigerants overview – most common

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AGENDA

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Agenda • Alternative processes

– Absorption, Ejector , Stirling , Electric

– Magnetic

• Two stage vapour compression cycles

– Open intercooler

– Closed intercooler

– Cascade

– Injection/economizer

– Transcritical CO2 cycles

• Exercise on two stage cycle with open intercooler

• Assignments for lecture 3

– 2-stage cycle in EES, R404A

• Summary

• Handouts

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ALTERNATIVE PROCESSES

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Objective

• Most refrigeration systems use vapour compression cycles with mechanical compressors

• Brief introduction to alternative refrigeration processes: – Absorption

– Ejector

– Stirling

– Electric (Peltiér)

– Magnetic

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Absorption

• Mixture used as fluid

– NH3 + H2O

– LiBr + H2O

• Requires heat for generator

• Requires power to pump

Kondensator

Fordamper

Pumpe

Generator

QH

Qom

QL

Termisk kompressor

Absorber

Intern

varmeveksler

Qom

Thermal compressor

Generator

Internal HX

Pump

Absorber

Condenser

Evaporator

Low efficiency But interesting when

waste heat at high temperatures…

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Ejector

• Typically water

• Requires heat added

EjektorKondensator

Fordamper

Pumpe Generator

QHQom

QL

Termisk kompressorThermal compressor

Pump Generator

Ejector Condenser

Evaporator

Difficult to produce ejector…

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Stirling

• www.sunpower.com www.globalcooling.com

http://www.sunpower.com/

http://www.globalcooling.com/

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Electric (Peltiér)

Cold end

Hot end

+

-

Low efficiency…

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Magnetic refrigeration

• Magnetic Refrigeration at Room Temperature (IIR Bulletin 2007-5)

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TWO STAGE CYCLES AND CASCADE SYSTEMS

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Objective

• Learn how to show processes in

– Piping and Instrument Diagrams (PI-D)

– Cycles in log(p),h diagram

• Establish mass and energy balances for model purposes

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Why 2-stage vapour compression cycle? • Refrigeration for low temperature applications

• Depending of the compressor and the refrigerant used two factors put a limit on the practical use of 1-stage compression when the pressure ratio p2/p1 becomes “too high”

– Discharge temperature (too high)

– Volumetric efficiency (decreases)

• Isentropic efficiency also decreases

• For ammonia a pressure ratio p2/p1 of 5-6 is “too high”

• For halocarbon refrigerants (HCFC, HFC) a pressure ratio p2/p1 of approx. 15 is “too high”

• 2-stage cycles offer possibilities of optimizing the energy efficiency

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Isentropic efficiency

• At large pressure ratios, the efficiency of compressors decrease

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Two stage cycles

• At large temperature lifts, discharge temperature gets to high:

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Two stage with open intercooler

Simple… Risk of gas in liquid line to low Temperature evaporator due to

pressure drop in long pipes

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Two stage with closed intercooler

No risk of gas in liquid line to low Temperature evaporator due to pressure drop in long pipes. Heat exchanger in vessel means lower efficiency (COP)

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Finding compressor sizes

• Very often most problems are solved by placing control volume around intercooler:

1 8

1

,

:

e L

LL

vol L

Low pressure

Q m h h

m vV

2 6 7 3

1 2 7 8

3 4 5 6

2 6 7 3

3

,

:

:

:

L

H

L H L H

HH

vol H

Intercooler

Massbalance

m m m m

Sationary

m m m m m

m m m m m

Energybalance

m h m h m h m h

m vV

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Finding compressor sizes

• It turns out that everything is typically known if the intermediate pressure is known!

• If both compressors are ideal, the optimal intermediate pressure is found as:

• I.e. this (design) pressure will give the highest COP if both compressors are ideal (and will also give the size of the compressors…)

• On the other hand: if you know compressor swept volumes and operating conditions, you will be able to calculate the resulting intermediate pressure…

m e cP P P

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Intermediate pressure

• Other normal used formulas for design intermediate pressure:

0.35

, [ ]

2

m e c

cm e c

e

e cm

P P P bar

TP P P temperatures in K

T

T TT

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Cascade

• Two different refrigerants

• Two 1-stage systems in sequence

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Example of industrial equipment

• Cascade system with R744/R717 (CO2/ammonia)

– High volumetric refrigerating effect of CO2 at low temperature (fewer compressors required)

– Only CO2 in production areas (safety)

– CO2 condensed by ammonia, no problem with critical pressure

– “Penalty” of using heat exchanger

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Supermarket cycles

• Two one stage with common condenser

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Supermarket cycles

• Two stage transcritical

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Transcritical – parallel compression

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Supermarket cycles

• Cascade CO2/ HFC or ammonia

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Language

• Economizer

– Injection of gas, liquid or mixture at strategic position in compressor (screws, two-stage recip,…)

– Sometimes economizer also covers two-stage with open intercooler (intercooler is called economizer…)

• Booster

– Booster compressor is the low-stage compressor

– Booster cycle is (sometimes) two-stage with liquid injection…

• CoolPack

– This software tools provides you with an extensive number of cycles

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EXERCISE

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Exercise 1

13

2

5

15

14

1

3

CYCLE ANALYSIS: TWO-STAGE CYCLE

T4 :

T1 :

T2 :

TE,HS :

TC :

X5 :

QE,HS :

QC :

WHS :

6

119

8

7

12

4

QE,LS :

WLS :

X9 :

T14 :

T13 :

34,0 [°C]

129,0 [°C]

-33,5 [°C]

-9,0 [°C]

-10,0 [°C]

35,0 [°C]

TE,LS : -35,0 [°C]

0,08 [kg/kg]

0,16 [kg/kg]

200,0 [kW]

200,0 [kW]

548,5 [kW]

34,4 [kW]

128,8 [kW]

1,250 [-]

1,250 [-]nCIRC,LS :

nCIRC,HS :

10

60,4 [°C]

mTOT : 0,396 [kg/s]

mCIRC,HS :

mCIRC,LS : 0,181 [kg/s]

0,192 [kg/s]

> FLOODED EVAPORATORS, OPEN INTERCOOLER, ONE-STAGE COMPRESSORS

SUBDIAGRAM

WINDOWS

REFRIGERANT : R717 COP :COP*HS :

COP*LS :2,452

nCARNOT,HS :

nCARNOT,LS :

0,57

0,61

3,359

5,833

CoolPack

Department of

Mechanical Engineering

Technical Univ ersity

TOOL C.5

© 1999 - 2001

of Denmark

Version 1.46

LOG(p),h-DIAGRAM

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Exercise 2

1. Draw the log(p)-h diagram for the 2-stage cycle with open intercooler (intermediate pressure pm = p3)

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Exercise 2 solution

1

2 3

4 5

6 7

8

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EXERCISE WITH EES

Assignment (see “Black Board” under lecture 3)

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Summary

• Heard about alternative processes like absorption, ejector, Stirling, electric, and magnetic.

• Studied two stage vapour compression cycles and cascade systems looking at PI-diagram, log(p)-h diagram, mass and energy balances

• Exercise on two stage cycle with open intercooler

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Handouts

• PDF files to be found on “Black Board”

– Lecture 3. Assignment

– R404A Log(p)-h diagram

Lecture 3 PDA 2011

Documents

Transcript of Lecture 3 PDA 2011