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

2. 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

3. Proportional Controllers
x is controller output
A is controller output with no error
(often A=0)
Kis proportional gain constant
e = is error (offset)

4. Unstable system
Stable system

5. 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

6. Proportional + Integral (PI)
K/Ti is integral gain
If controller is tuned properly, offset is reduced to zero
Figure 2-18a

8. Issues with PI Controllers
Scheduling issues
Require more tuning than for P
But, no offset

9. 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

10. Ref: Kreider and Rabl.Figure 12.5

11. 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”

12. 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

13. 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

14. Relative humidity control by cooling coil
Mixture
Room
Supply
TDP
Heating coil

15. 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

16. 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

17. Absorption Cycle Same as vapor compression but NO COMPRESSOR
Replace compressor

18. 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

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

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

21. 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

22. 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

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

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

25. 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)