1. Lecture Objectives: Discuss HW4, answer your questions
Finish with desiccant systems
Learn about cooling towers

2. Heat recovery system in NSM building at UT
Sensible W
Enthalpy W

3. HW4 (Due March 23rd)
1) Using NMS’ AHU, show the process in the Psychrometric chart for Austin’s
a) Fall humid day (Outdoor: T=24C & RH=90%)
b) Fall dry day (Outdoor: T=24C & RH=50%)
c) Winter (T=3C RH=70%)
For fall: T hot deck is minimum 24C, T cold deck is 13C
For winter: T hot deck is 35C, T cold deck is 13C
For each case: (a,b, and c) calculate RH of supply air in hot and cold deck.

4. Desiccant wheel

5. Desiccant wheel
Figure 3 – A desiccant-based cooling system combined with regenerative heat exchanger, vapor compression cooling, and evaporative humidifier (hybrid system).

6. Variation in Cycles
Much more in the paper I gave you (Technical development of rotary
desiccant dehumidification and air conditioning)

7. Cooling Tower
Similar to an evaporative cooler, but the purpose is often to cool water
Widely used for heat rejection in HVAC systems
Also used to reject industrial process heat

9. Cooling Tower

10. Various Use For small buildings For power plant (all kinds)
One of the cooling tower at UT
For industrial application

12. 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

13. Coupling of cooling tower with the water cooled condenser

14. System Modeling

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

16. Function fitting for a chiller q = f (condensing and evaporating T)

17. 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

18. 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

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

20. 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.

21. 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)

22. 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().