Tài liệu Hvac sequence of operation

HVAC Systems at Y2E2 Y2E2 – HVAC sequence of operation (controls) Week 3 CEE 243 Copyright  2012 HVAC Systems at Y2E2 Big idea • Modern energy management systems include a combination of passive and active control of energy distribution – Passive: constant (volume/flow) – Active: variable (volume/flow) under computer control • Based on If-then conditional control rules of general form If setpoint value out of range and Other conditions met Then adjust control variable value CEE 243 2 Copyright  2012 HVAC Systems at Y2E2 Jargon • • • • • • • • • • Active beam AHU CAV Cfm Condition Diffuser Diff pressure Fabric loss Fan coil Heat exchanger • • • • • • • • • JK-1 Natural ventilation Radiant slab Setpoint Tempered hot/cold water U value VAV Zone Zone splitter CEE 243 3 Copyright  2012 HVAC Systems at Y2E2 Control basics • Typically active response components have a setpoint, i.e., operating functional objective value – E.g., room temperature setpoint = 73 +- 3oF • Under computer control, the HVAC building management system (BMS) adjusts value of active control components to maintain the setpoint of response components – e.g., flow rate damper position = 0 – 100% open • BMS rules “tell” control system how to reach setpoint • BMS is linked to input sensors and output control signals CEE 243 4 Copyright  2012 HVAC Systems at Y2E2 Active beams provide efficient conditioned fresh air to spaces: mostly passive (Chilled) water passes through "beam" heat exchanger  beam chills air  air becomes denser  (cool) air falls to floor  Convection & room cooling • Heating similar Reason for efficiency: heat capacity of water >> that of air: 1 cubic foot • Air: heat capacity 37 JK-1 • Water: heat capacity 20,050 JK-1.  heat/cool with much lower pumped volume in smaller components CEE 243 5 Copyright  2012 HVAC Systems at Y2E2 Simple control example – Active beam constant Heat if SP < 71 F Cool if SP > 75 F Deadband +/- 2 F CEE 243 6 Copyright  2012 HVAC Systems at Y2E2 (Direct) Response variable Deadband control 1000 120 900 100 700 Air flow CFM 80 600 500 400 60 Deadband 40 300 Valve Position percent 800 Control variable Air Flow Cooling Valve Heating Valve 200 20 100 0 0 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 Temperature degrees F CEE 243 Setpoint 7 Copyright  2012 HVAC Systems at Y2E2 Active beams • Active (control) components: Cold and hot water valves, air flow damper • Temperature control examples: – Occupied room air temperature setpoint user adjustable +/- 3F – Unoccupied setpoints: cooling =78F; heating = 65F – Control cold and/or hot water active beam valve position to achieve room air temperature setpoint – Temperature control deadband of +/- 2F – User override can extend occupied hours by 2 hours • Active beam air flow control: – Air flow rate constant on during occupied hours (basement 24/7); off during unoccupied hours – CAV boxes modulate flow to achieve constant air flow during occupied hours only, except during night setback and warmup (compensating for different air pressures) CEE 243 8 Copyright  2012 HVAC Systems at Y2E2 North facing spaces contain radiators to balance fabric heat losses Fabric heat loss through materials w/ temperature difference, e.g., walls, windows, floor, roof. Heat loss Q = ‘U’ * Area * Delta-temperature CEE 243 9 Copyright  2012 HVAC Systems at Y2E2 Baseboard radiators • Active components: hot water control valve • Temperature control examples: – Occupied temperature setpoint adjustable by user +/- 3F – Unoccupied heating temperature setpoint: 65F – Control hot water valve position of baseboard heater to achieve room air temperature setpoint – If outside air temperature > 78F  close valve at all times – User override extends occupied hours 2 hours CEE 243 10 Copyright  2012 HVAC Systems at Y2E2 Conference rooms • Active beam control as in other rooms • Active component: inline supply fan • Additional CO2 level control – If CO2 concentration > max CO2 setpoint  open inline supply fan – Max CO2 setpoint = 500 ppm above outside air CO2 concentration CEE 243 11 Copyright  2012 HVAC Systems at Y2E2 Laboratories • Active components: damper, water coil valves • Air flow control – The greater of: • The required air changes (6 air changes/hr) • Amount needed for cooling • Sufficient makeup air for fume hoods (supply ~ exhaust) – For heating air flow is set to minimum – For cooling air flow is set to maximum • Temperature control – +/- 2F deadband CEE 243 12 Copyright  2012 HVAC Systems at Y2E2 Radiant floor CEE 243 13 Copyright  2012 HVAC Systems at Y2E2 Radiant floor • Active components: Pump and valve • Pump off if outside air temperature > 78F • Otherwise – Turn on pump – To reach steady state: • Increase valve position by 10% (every 10 min) until temperature setpoint of 71F is met • Decrease valve position by 5% (every 10 min) until temperature setpoint of 71F is met – After reaching steady state • Adjust valve position by 1% (every 10 min) only • During unoccupied hours temperature setpoint = 66F CEE 243 14 Copyright  2012 HVAC Systems at Y2E2 Fan Coil Units CEE 243 15 Copyright  2012 HVAC Systems at Y2E2 Fan coil units • Active components: chilled water control valve • On 24/7 • Temperature control: – Modulate chilled water valve position to achieve temperature setpoint if space temperature is above setpoint – If two spaces are controlled by one unit, control using higher space temperature CEE 243 16 Copyright  2012 HVAC Systems at Y2E2 Main chilled water loop Serves: • Tempered water loop • Cooling coils (AHU, FC, CVs, VAVs) • Server Rack CEE 243 17 Copyright  2012 HVAC Systems at Y2E2 Chilled water system • Active components: Control valve and booster pump • Normal operation: Valve is used to control differential pressure to setpoint (if not enough booster pump helps out) • If water return temp < 58F (for > 5 min) reset down differential pressure setpoint • If water return temp < 55F (for > 5 min), start pump and recirculate water – If water supply temp < 48F go back to normal CEE 243 18 Copyright  2012 HVAC Systems at Y2E2 Tempered chilled water loop Serves: • Active beams CEE 243 19 Copyright  2012 HVAC Systems at Y2E2 Tempered chilled water system • Active components: Two pumps • On/off speed control – Lead/lag operation (weekly) – Pumps are controlled to meet differential pressure setpoint • lead pumps first • If pressure difference too small, lag pump starts in addition • If both run slow (< 25Hz), turn off lag pump – Pumps should be off during unoccupied hours (expect overrides) • Temperature control – Maintain supply water temperature at 60F by opening and closing control valve • Supply Temp Reset (to prevent condensation on active beams) – If outside dewpoint temperature > 58F -> supply temperature setpoint = dewpoint temperature + 2F CEE 243 20 Copyright  2012