Objectives • Control Terminology • Types of controllers – Differences • Controls in the real world – Problems – Response time vs. stability Motivation • Maintain environmental quality – Thermal comfort – Indoor air quality – Material protection • Conserve energy • Protect equipment Basic purpose of HVAC control Daily, weekly, and seasonal swings make HVAC control challenging Highly unsteady-state environment Provide balance of reasonable comfort at minimum cost and energy Two distinct actions: 1) Switching/Enabling: Manage availability of plant according to schedule using timers. 2) Regulation: Match plant capacity to demand History • • • • • Process controls Self-powered controls Pneumatic and electro-mechanical controls Electronic controls Direct digital control (DDC) Terminology • Sensor – Measures quantity of interest • Controller – Interprets sensor data • Controlled device – Changes based on controller output Figure 2-13 outdoor Direct Indirect Closed Loop or Feedback Open Loop or Feedforward • Set Point – Desired sensor value • Control Point – Current sensor value • Error or Offset – Difference between control point and set point Two-Position Control Systems • Used in small, relatively simple systems • Controlled device is on or off – It is a switch, not a valve • Good for devices that change slowly • Anticipator can be used to shorten response time • Control differential is also called deadband Residential system - thermostat • ~50 years old DDC thermostat - Daily and weekly programming Modulating Control Systems Example: Heat exchanger control – Modulating (Analog) control Cooling coil air x water (set point temperature) Modulating Control Systems • Used in larger systems • Output can be anywhere in operating range • Three main types – Proportional – PI – PID Electric (pneumatic) motor Position (x) fluid Volume flow rate Vfluid = f(x) - linear or exponential function The PIDconstants control algorithm time e(t) – difference between set point and measured value Position (x) Proportional Integral Differential For our example of heating coil: d (Tset point Tmeasured ) K x K (Tset point Tmeasured ) (Tset point Tmeasured )d K Td Ti d Proportional (how much) Position of the valve Integral (for how long) Differential (how fast) Proportional Controllers x A K (Tset point Tmeasured ) x is controller output A is controller output with no error (often A=0) Kis proportional gain constant e = Tset point Tmeasured is error (offset) Unstable system Stable system Issues with P Controllers • Always have an offset • But, require less tuning than other controllers • Very appropriate for things that change slowly – i.e. building internal temperature Proportional + Integral (PI) K x A K (Tset point Tmeasured ) (Tset point Tmeasured )d Ti K/Ti is integral gain If controller is tuned properly, offset is reduced to zero Figure 2-18a Issues with PI Controllers • Scheduling issues • Require more tuning than for P • But, no offset Proportional + Integral + Derivative (PID) • Improvement over PI because of faster response and less deviation from offset – Increases rate of error correction as errors get larger • But – HVAC controlled devices are too slow responding – Requires setting three different gains Ref: Kreider and Rabl.Figure 12.5 The control in HVAC system – only PI x K (Tset point Tmeasured ) K (Tset point Tmeasured )d Ti Proportional Integral value Set point Set point Proportional affect the slope Integral affect the shape after the first “bump” The Real World • 50% of US buildings have control problems – 90% tuning and optimization – 10% faults • 25% energy savings from correcting control problems • Commissioning is critically important Practical Details • • • • • Measure what you want to control Verify that sensors are working Integrate control system components Tune systems Measure performance Commission control systems HVAC Control Example 1: 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 Economizer % fresh air Fresh air volume flow rate control enthalpy damper Fresh (outdoor) air TOA (hOA) 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 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 (ECJ research facility) 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