Lecture Objectives: - University of Texas at Austin

Lecture Objectives: - University of Texas at Austin

Lecture Objectives: -Answer questions related to HW 4 - Solar Systems -Continue with HVAC system Building HVAC Systems (Primary and Secondary Building HVAC Systems) AHU Air Handling Unit Fresh air for ventilation AHU Primary systems Distribution systems

Air transport Electricity Cooling (chiller) (or Gas) Heating (boilers) Gas Secondary systems Building envelope HVAC systems affect the energy efficiency of the building as much as the building envelope. In many situation even more!

Examples of HVAC System Multizone Dual Duct System Multi zone VAV with Re-heaters 55F 90F 55F P P C Perimeter (P) Core (C) C Dual Duct vs. VAV with Re-heaters

for Different Weather Conditions What happens if outdoor air is A, B, C A B C Example of a Plant System (Chilled Water System) Air cooled chiller Chiller with a cooling tower COP ~ 3 COP = Cooling Energy / Electric Energy COP ~ 5

( same units) Two Basic Approaches for Modeling of HVAC and Building Envelope Load System Plant model Qbuiolding Building Heating/Cooling System Q Plant including Ventilation and Dehumidification Integrated models

Building Heating/Cooling System Plant Example of a HVAC Model Schematic of simple air handling unit (AHU) ca M R w O h _ b 1 -r)m C

(Q S o f,in T tlse u H Mixing box m - mass flow rate [kg/s], T temperature [C], w [kgmoist/kgdry air], r - recirculation rate [-], Q energy/time [W] Example of a Plant Models (Chiller) P electric () = COP () x Q cooling coil () What is COP for this air cooled chiller ? TOA

T Condensation = TOA+ T Evaporation at 1oC o water TCWS=5 C TCWR=11oC Building users (cooling coil in AHU) COP is changing with the change of TOA Plant model Refrigeration Cycle Released energy (condenser) T outdoor air

T cooled water - What is COP? - How the outdoor air temperature affects chiller performance? Cooling energy (evaporator) Chiller model: COP= f(TOA , Qcooling , chiller properties) Chiller data: QNOMINAL nominal cooling power, PNOMINAL electric consumption for QNOMINAL The consumed electric power [KW] under any condition P PNOMINAL CAPFT EIRFT EIRFPL Available capacity as function of evaporator and condenser temperature Cooling water supply

Outdoor air 2 2 CAPTF a1 b1 TCWS c1 TCWS d1 TOA e1 TOA f1 TCWS TOA Full load efficiency as function of condenser and evaporator temperature 2 2 EIRFT a2 b2 TCWS c2 TCWS d 2 TOA e2 TOA f 2 TCWS TOA Efficiency as function of percentage of load EIRFPLR a3 b3 PLR c3 PLR Percentage of load: PLR

The coefficient of performance under any condition: COP( ) Q( ) P ( ) Q( ) QNOMINAL CAPFT Models integrated in HVAC System simulation Example: Economizer (fresh air volume flow rate control) Controlled device is damper damper fresh air - Damper for the air - Valve for the liquids

mixing recirc. air T & RH sensors HVAC Control Economizer (fresh air volume flow rate control) Controlled device is damper damper fresh air - Damper for the air - Valve for the liquids mixing recirc.

air % fresh air T & RH sensors 100% Minimum for ventilation Economizer cooling regime How to control the fresh air volume flow rate? If TOA < Tset-point Supply more fresh air than the minimum required The question is how much? % fresh air Open the damper for the fresh air and compare the Troom with the Tset-point . Open till you get the Troom = Tset-point If you have 100% fresh air and your

still need cooling use cooling coil. 100% Minimum for ventilation What are the priorities: - Control the dampers and then the cooling coils or - Control the valves of cooling coil and then the dampers ? Defend by SEQUENCE OF OERATION the set of operation which HVAC designer provides to the automatic control engineer

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