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Objectives

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- Finish DOAS
- Control
- Terminology
- Types of controllers
- Differences

- Controls in the real world
- Problems
- Response time vs. stability

Fresh air?

- Split of sensible and latent load
- Selection of hydronic system
- Winter vs. summer operation
- Winter operation with DX systems (heat pump)

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

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

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

- 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

- 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

- 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

- DDC thermostat
- Daily and weekly
- programming

- ~50 years old

Example: Heat exchanger control

Modulating (Analog) control

Cooling coil

air

water

x

(set point temperature)

Electric (pneumatic) motor

Position (x)

fluid

Volume flow rate

Vfluid = f(x) - linear or exponential function

- Used in larger systems
- Output can be anywhere in operating range
- Three main types
- Proportional
- PI
- PID

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

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

- Always have an offset
- But, require less tuning than other controllers
- Very appropriate for things that change slowly
- i.e. building internal temperature

- K/Ti is integral gain

If controller is tuned properly, offset is reduced to zero

Figure 2-18a

- Scheduling issues
- Require more tuning than for P
- But, no offset

- 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

Proportional

Integral

value

Set point

Proportional

affect the slope

Integral

affect the shape after

the first “bump”

Set point

- 50% of US buildings have control problems
- 90% tuning and optimization
- 10% faults

- 25% energy savings from correcting control problems
- Commissioning is critically important