1. Lecture Objectives: Continue with cooling towers
Learn about modeling of complex HVAC systems
Discuss Project 1 assignment

2. Cooling Tower

3. Modeling of Heat and Mass Transfer

4. Cooling Tower Performance Curve
TCTR
Outdoor
WBT
from chiller
TCTS
to chiller
Temperature difference:
R= TCTR -TCTS
TCTS
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)
WBT

5. Coupling of cooling tower with the water cooled condenser

6. Analytical Modeling of a Chiller (example of a simplified model of a heat exchanger)

7. System Modeling (Empirical)

8. Example: Modeling of a pump

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

10. Function fitting for a Chiller q = f (condensing and evaporating T)

11. Cooling Tower Performance Curve
TCTR
Outdoor
WBT
from chiller
TCTS
to chiller
Temperature difference:
R= TCTR -TCTS
TCTS
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)
WBT

12. 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
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

13. Modeling of Water Cooled Chiller
(COP=Qcooling/Pelectric)
Chiller model:
COP= f(TCWS , TCTS , Qcooling , chiller properties)

14. Modeling of Water Cooled Chiller
Chiller model:
Chiller data: QNOMINAL nominal cooling power,
PNOMINAL electric consumption for QNOMINAL
Available capacity as function of evaporator and condenser temperature
Cooling water supply
Cooling tower supply
Full load efficiency as function of condenser and evaporator temperature
Efficiency as function of percentage of load
Part load:
The consumed electric power [KW] under any condition of load
The coefiecnt of performance under any condition
Reading: page 597.

15. Combining Chiller and Cooling Tower Models
Function of TCTS
3 equations from previous slide
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)

16. Merging Two Models Temperature difference: R= TCTR -TCTS 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
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().

17. Low Order Building Modeling
Measured data or
Detailed modeling
Find Q() = f (DBT)

18. For Austin’s Office Building
Model: (Area = 125,000sf)
Hours in a year kW
Used for component
capacity analysis
Model
=0 when building is off
Number of hours

19. For project 1 you will need Q() for each hour
Yearly based analysis:
You will need Q() for one week in July
Use simple molded below and the Syracuse TMY2 weather file posted in the course handout section 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 4 8 12 16
Q= *t
Q= *t
Q [ton]
t [F]