Lecture Objectives: Discuss HW2 Finish with HVAC control (PID loops, Sequence of operation) Learn about sorption chillers

Modulating Control Systems Used in larger systems Output can be anywhere in operating range Three main types Proportional PI PID Position fluid Electric (pneumatic) motor Vfluid = f(position) - linear or exponential function Volume flow rate

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

Ref: Kreider and Rabl.Figure 12.5

The control in HVAC system – only PI Proportional Integral value Set point Proportional affect the slope Set point Integral affect the shape after the first “bump”

The Real World 50% of US buildings have control problems 90% tuning and optimization 10% faults Huge energy savings from correcting control problems Commissioning is critically important

HVAC Control Example : Dew point control (Relative Humidity control) fresh air damper filter cooling coil heating coil filter fan 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 Mixture Room Supply TDP 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 research facility) Set Point (SP) Mixture 2 Mixture 3 Mixture 1 DBTSP DPTSP 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) 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

Absorption Cycle Same as vapor compression but NO COMPRESSOR Replace compressor

Absorption cooling cycle Relatively simple thermodynamics with adition of mixtures (water – aminia) Rich solution of Heat H2O H2O + NH3 Rich solution of H2O H2O + NH3

Mixtures (T-x diagram) Dew point curve Saturated vapor Mixture of liquid and vapor Saturated liquid Bubble point curve For P= 4 bar

h-x diagram hfg hfg Isotherms are showmen only in liquid region for H2O hfg for NH3 Isotherms are showmen only in liquid region

Composition of h-x diagram Saturated vapor line at p1 Equilibrium construction line at p1 1e Used to determine isotherm line in mixing region! Start from x1; move up to equilibrium construction line; move right to saturated vapor line; determine 1’; connect 1 and 1’. Isotherm at P1 and T1 Adding energy B A x1 X1’ mass fraction of ammonia in saturated vapor

h-x diagram at the end of your textbook you will find these diagrams for 1) NH3-H2O 2) H2O-LiBr LiBr is one of the major liquid descants in air-conditioning systems

Adiabatic mixing in h-x diagram (Water – Ammonia) From the textbook (Thermal Environmental Eng.; Kuehen et al)

Absorption cooling cycle Rich solution of Heat H2O H2O + NH3 Rich solution of H2O H2O + NH3

Mixing of two streams with heat rejection (Absorber) mixture of H2O and NH3 m2 m3 =pure NH3 (x2=1) m1 m3 m2 m1 2 Q cooling Heat rejection Mixture of 1 and 2 3’ Mass and energy balance: (1) (2) 1 3 (3) x3 x From mixture equation: Substitute into (2) Substitute into (3) From adiabatic mixing (from previous slide)