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## Objectives

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**Objectives**• Finish DOAS • Control • Terminology • Types of controllers • Differences • Controls in the real world • Problems • Response time vs. stability**Issues Related to DOAS**• Split of sensible and latent load • Selection of hydronic system • Winter vs. summer operation • Winter operation with DX systems (heat pump)**Sequence of operation for the control system design**Adiabatic humidifier CC HC SA OA mixing RA Define the sequence of operation for: WINTER operation and: - case when humidity is not controlled - case when humidity is precisely controlled Solution on the whiteboard**Economizer**Fresh air volume flow rate control % fresh air 100% enthalpy Fresh (outdoor) air TOA (hOA) Minimum for ventilation damper mixing Recirc. air T & RH sensors**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**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**Terminology**• Sensor • Measures quantity of interest • Controller • Interprets sensor data • Controlled device • Changesbased on controller output Figure 2-13**outdoor**Direct Closed Loop or Feedback Indirect 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**• DDC thermostat • Daily and weekly • programming • ~50 years old**Example: Heat exchanger control**Modulating (Analog) control Cooling coil air water Modulating Control Systems x (set point temperature)**Electric (pneumatic) motor**Position (x) fluid Volume flow rate Vfluid = f(x) - linear or exponential function Modulating Control Systems • Used in larger systems • Output can be anywhere in operating range • Three main types • Proportional • PI • PID**The PID control algorithm**For our example of heating coil: constants time e(t) – difference between set point and measured value Position (x) Differential Proportional Integral Differential (how fast) Proportional (how much) Integral (for how long) Position of the valve**Proportional Controllers**x is controller output A is controller output with no error (often A=0) Kis proportional gain constant e = 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/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**The control in HVAC system – only PI**Proportional Integral value Set point Proportional affect the slope Integral affect the shape after the first “bump” Set point**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