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

Cooling Tower

Modeling of Heat and Mass Transfer

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

Coupling of cooling tower with the water cooled condenser

Analytical Modeling of a Chiller (example of a simplified model of a heat exchanger) https://eetd.lbl.gov/sites/all/files/engineeringreference.pdf

System Modeling (Empirical)

Example: Modeling of a pump

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

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

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

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

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

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

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)

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().

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

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

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=-0.45 +0.0448*t Q=--27.48+0.5152*t Q [ton] t [F]