Lecture Objectives: •Summarize sorption chillers •Learn about •Chiller modeling •Cooling towers and modeling
Jan 01, 2016
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]