Lecture Objectives:

• Summarize sorption chillers • Learn about

• Chiller modeling• Cooling towers and modeling

Example of H2O-NH3 System

• Text Book (Thermal Environmental Engineering) Example 5.5

• HW 4:• Solve the problem 5.6 (water – ammonia)

• from the textbook

• Based on example 5.5.• You may need to study example 5.6 and 5.7• Due date is next Tuesday

LiBr-H2O Systems

LiBr-H2O Systems

Twine vessel LiBr-H2O Systems

System with no pump(Platen-Munter system)

• H2O-NH3 + hydrogen

http://www.youtube.com/watch?v=34K61ECbGD4

Useful information about LiBr absorption chiller

• http://www.cibse.org/content/documents/Groups/CHP/Datasheet%207%20-%20Absorption%20Cooling.pdf

Practical Tips for Implementation of absorption chillers • Identify and resolve any pre-existing problems with a cooling system, heat rejection

system, water treatment etc, before installing an absorption chiller, or it may be unfairly blamed.

• Select an absorption chiller for full load operation (by the incorporation of thermal stores if necessary) as COP will drop by up to 33% at part-load.

• Consider VSD control of absorbent pump to improve the COP at low load. • Consider access and floor-loading (typical 2 MW Double-effect steam chiller 12.5 tons

empty, 16.7 tones operating). • Ensure ambient of temperature of at least 5°C in chiller room to prevent crystallization. • http://www.climatewell.com/index.html#/applications/solar-cooling

Central chiller plant

Modeling of Water Cooled Chiller

(COP=Qcooling/Pelectric)

Chiller model:

COP= f(TCWS , TCTS , Qcooling , chiller properties)

Example of a vapor compression chiller

Modeling of Water Cooled Chiller

CTSCWSCTSCTSCWSCWS TTfTeTdTcTbaCPATF 12

112

111

Chiller model:

Cooling water supply Cooling tower supply

Available capacity as function of evaporator and condenser temperature

CTSCWSCTSCTSCWSCWS TTfTeTdTcTbaEIRFT 22

222

222

Full load efficiency as function of condenser and evaporator temperature

PLRcPLRbaEIRFPLR 333

Efficiency as function of percentage of load

CAPFTQ

QPLR

NOMINAL

)(Part load:

The coefiecnt of performance under any condition

EIRFPLEIRFTCPFTPP NOMINAL )(

)()(

P

QCOP

Chiller data: QNOMINAL nominal cooling power, PNOMINAL electric consumption for QNOMINAL

The consumed electric power [KW] under any condition of load

Reading: http://apps1.eere.energy.gov/buildings/energyplus/pdfs/engineeringreference.pdf page 597.

Example of a chiller model

http://www.comnet.org/mgp/content/chillers?purpose=0

Combining Chiller and Cooling Tower Models

EIRFPLEIRFTCPFTPP NOMINAL

3 equations from previous slide

Function of TCTS

22444

2444

2444 ][][ RWBTiWBThgRWBTfWBTedWBTcWBTbaTCTS

Add your equation for TCTS

→ 4 equation with 4 unknowns (you will need to calculate R based on water flow in the cooling tower loop)

Merging Two Models

Finally: Find P() or

The only fixed variable is TCWS = 5C (38F) and Pnominal and Qnominal for a chiller (defined in nominal operation condition: TCST and TCSW); Based on Q() and WBT you can find P() and COP().

Temperature difference:

R= TCTR -TCTS

22444

2444

2444 ][][ RWBTiWBThgRWBTfWBTedWBTcWBTbaTCTS

Model:

Link between the chiller and tower models is the Q released on the condenser: Q condenser = Qcooling + Pcompressor ) - First law of Thermodynamics

Q condenser = (mcp)water form tower (TCTR-TCTS) m cooling tower is given - property of a tower

TCTR= TCTS - Q condenser / (mcp)water

)(

)()(

P

QCOP

Cooling Towers

Power plant type

Major difference: NO FAN

Cooling Tower Performance Curve

Most important variable is wet bulb temperature

TCTS = f( WBToutdoor air , TCTR , cooling tower properties)

or for a specific cooling tower type

TCTS = f( WBToutdoor air , R)

from chillerOutdoor WBT

TCTS

R

Temperature difference:

R= TCTR -TCTS

TCTR

to chiller

WBT

TCTS

Cooling Tower Model Model which predict tower-leaving water temperature (TCTS) for arbitrary entering water temperature (TCTR) and outdoor air wet bulb temperature (WBT)

Temperature difference:

R= TCTR -TCTS

22444

2444

2444 ][][ RWBTiWBThgRWBTfWBTedWBTcWBTbaTCTS

Model:

For HW 3b:

You will need to find coefficient a4, b4, c4, d4, e4, f4, g4, h4, and i4 based on the graph from the previous slide and two variable function fitting procedure

Two variable function fitting(example for a variable sped pump)

Function fitting for a chillerq = f (condensing and evaporating T)

180 2 4 6 8 10

0

50

100

150

20025 C35 C45 C

q[kW]

Tevaporator [C]