VAV Minimum Air Selection VAV Example One system

VAV Minimum Air Selection

VAV Example -One system -Multiple Spaces/Zones - VAV box for each space/zone http: //www. trane. com/commercial/library/vol 242/v 24 b 6. asp

VAV System Terms & Equations (you can find them in ASHRAE Standard 62) • Fraction of minimum outdoor air in space/zone supply air Xi = VOAi / VSAi • Critical space Space with maximum Xi , Z=max(Xi) • Fraction of outdoor air in system supply air X = sum(VOA ) / sum(VSA) • Required Fraction of outdoor air: Y=X/(1+X-Z)

Example Solution Space 1 VSA VOA Xi X Z Y Space 2 Space 3 Space 4

Examples of sequence of operation

HVAC Control Example 1: Economizer (fresh air volume flow rate control) Controlled device is damper fresh air - Damper for the air - Valve for the liquids mixing recirc. air T & RH sensors

Economizer % fresh air Fresh air volume flow rate control enthalpy damper Fresh (outdoor) air TOA (h. OA) mixing Recirc. 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. 100% Open till you get the Troom = Tset-point If you have 100% fresh air and your still need cooling use cooling coil. 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

Economizer – cooling regime Example of SEQUENCE OF OERATIONS: If TOA < Tset-point open the fresh air damper the maximum position Then, if Tindoor air < Tset-point start closing the cooling coil valve If cooling coil valve is closed and T indoor air < Tset-point start closing the damper till you get T indoor air = T set-point Other variations are possible

HVAC Control Example 2: Dew point control (Relative Humidity control) damper fan fresh filter cooling heating air coil filter mixing T & RH sensors Heat gains Humidity generation We should supply air with lower humidity ratio (w) and lower temperature We either measure Dew Point directly or T & RH sensors substitute dew point sensor

Relative humidity control by cooling coil Cooling Coil Mixture Supply TDP Room Heating coil

Relative humidity control by cooling coil (CC) • Cooling coil is controlled by TDP set-point if TDP measured > TDP set-point → send the signal to open more the CC valve if TDP measured < TDP set-point → send the signal to close more the CC valve • Heating coil is controlled by Tair set-point if Tair < Tair set-point → send the signal to open more the heating coil valve if Tair > Tair set-point → send the signal to close more the heating coil valve Control valves Fresh air mixing cooling coil heating coil Tair & TDP sensors

Sequence of operation (PRC environmental chamber) Mixture 3 Set Point (SP) Mixture 1 DPTSP Mixture 2 Control logic: DBTSP Mixture in zone 1: IF (( TM<TSP) & (DPTM<DPTSP) ) heating and humidifying Heater control: IF (TSP>TSA) increase heating or IF (TSP<TSA) decrease heating Humidifier: IF (DPTSP>DPTSA) increase humidifying or IF (DPTSP<DPTSA) decrease humid. Mixture in zone 2: IF ((TM>TSP) & (DPTM<DPTSP) ) cooling and humidifying Cool. coil cont. : IF (TSP<TSA) increase cooling or IF (TSP>TSA) decrease cooling Humidifier: IF (DPTSP>DPTSA) increase humidifying or IF (DPTSP<DPTSA) decrease hum. Mixture in zone 3: IF ((DPTM>DPTSP) ) cooling/dehumidifying and reheatin Cool. coil cont. : IF (DPTSP>DPTSA) increase cooling or IF (DPTSP<DPTSA) decrease cooling Heater control: IF (TSP>TSA) increase heating or IF (TSP<TSA) decrease heating