Objectives Discuss Project Topics Learn to design VAV and DOAS System

3D model – Biomedical Engineering (BME) Building

VAV Example -One system -Multiple Spaces/Zones - VAV box for each space/zone

VAV System Terms & Equations (you can find them in ASHRAE Standard 62) Fraction of minimum outdoor air in space/zone supply air X i = V OAi / V SAi Critical space Space with maximum X i, Z=max ( X i ) Fraction of outdoor air in system supply air X = sum(V OA ) / sum(V SA ) Required Fraction of outdoor air: Y=X/(1+X-Z)

Example Solution Space1Space 2Space 3Space 4 V SA V OA Xi X Z Y

Other Issues Related to VAV Fan control Minimum supply air Occupancy sensors

DOAS System

Enthalpy wheel

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

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

Economizer Fresh air volume flow rate control mixing damper Fresh (outdoor) air T & RH sensors Recirc. air % fresh air Minimum for ventilation 100% T OA (h OA ) enthalpy

Economizer – cooling regime How to control the fresh air volume flow rate? % fresh air Minimum for ventilation 100% If T OA < T set-point → Supply more fresh air than the minimum required The question is how much? Open the damper for the fresh air and compare the T room with the T set-point. Open till you get the T room = T set-point If you have 100% fresh air and your still need cooling use cooling coil. 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 T OA < T set-point open the fresh air damper the maximum position Then, if T indoor air < T set-point start closing the cooling coil valve If cooling coil valve is closed and T indoor air < T set-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) filter fan cooling coil heating coil filter mixing damper fresh air T & RH sensors We either measure Dew Point directly or T & RH sensors substitute dew point sensor Humidity generation Heat gains We should supply air with lower humidity ratio (w) and lower temperature

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

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

Sequence of operation (ECJ research facility) Control logic: 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) 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 Set Point (SP) Mixture 2 Mixture 3 Mixture 1 DBT SP DPT SP