HVAC Controls 2003

7/31/2019 HVAC Controls 2003

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HVAC System controls

By

Samantha Gunawardana

HVAC- Heating, ventilation and air conditioning-

account for 70 to 80 % of a buildings operating

cost

High energy prices are driving building owners to

seek automation and cut costs.

Demands for better air quality are increasing

through legislation.

Use systems only on Demand Control systems

Energy Share

Mar 2010 Apr 2010

May 2010


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Why do we need a control system?

Basic controls: Form of Manual switching

Time switches

Thermostats

The purpose of a control system is to automate

and take control of these operations in most

efficient way for the occupiers

Fundamentals of Automatic controls


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Typically there are two types of control loopsused in HVAC applications.




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The controllers can be classified by the type of controlaction taken when the controlled variable deviates

from the set point.


Most common

Wide temp variation

Rapid cycling

More stable and less cycling

than two-position control


The amount of change in the

controlled variable, over the

full range of controlled

device is called the throttlingrange

Provides stable control

Inherent disadvantage is its

offset.


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Integral control overcomes

the offset characteristics of

the proportional control.

It responds not only to

magnitude of the deviation,

but also for how long it

exists.


Derivative control generates

a corrective output signal

only when the condition of

controlled variable is

changing.


PI controls is used in

applications where offset is

unacceptable. Good for

places where no rapidchanges in controlled

variable. Good for HVAC

applications


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DDC Vs Conventional

Conventional controls are sometimes referred asSingle loop controls(SLC)

In Contrast Direct Digital Controls many control

sequences simultaneously.

DDC Vs Conventional

Control system Designer

Need to know about DDC

Need to know the at least basics of what equipment

to be controlled Related guidelines for those equipment

Must be a expert on every system that he or she

works with and seeks to control

Should be familiar with computer systems

Basics of electronics and electricity

Knowledge of control theory


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Input and Sensors

There are five specific type of sensors Temp Sensors(Bimetal, Rod and tube, RTD,

Thermistor, Thermocouple

Humidity sensors

Pressure sensors

Flow sensors

Special purpose sensor(watt, current, kw pulse,CO)

Input and Sensors

Sensor selection consideration

Environment

Operating range

Sensor signal compatibility

Set point accuracy and repeatability

Response time (Fast response is not always

desirable)

Controllers

Controller is a device that collects information from the sensor,

interprets this information, and send commands to controlled

device to effect a corrective action.

Types of controllers

Pneumatic

Electric

Electronic

Direct Digital

Hybrid


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Controlled Devices

Controlled devices are components that are controlled by

mechanical or electrical means

Common controlled devices are

Control Valves

Control Dampers

Control Valves(Two position, Proportional)

Mech engineer to design the constant pressure drop across the valve

There are three general classifications of flow characteristics

Equal percentage

Lineari ty

Quick Opening

Controlled Devices

Control Valves operator

To operate a motor control valve through its stroke a

controlled device called Actuator is needed.

Electrical solenoid (Plunger is connected to valve stem)

Electric Motor actuators (Utilise a gear train)

Birdirectional

Spring return

Reversible

Controlled Devices

Control Damper(A valve

for air)

Two Position Modulating control

Damper actuator

Rotation shaft designed to

link with damper

Electrical Auxiliary

devices


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Static pressure control

Purpose of static pressure control is to maintain minimum amount

of static pressure in duct system to support proper air distribution

Supply and return air flow control

Two primary modes of

controls are

Duct static pressure

Building static pressure


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Variable air volume control

The purpose of VAV terminal control is to modulate the cooling

supply air to meet the minimum cooling requirement of the space

Variable air volume control-contd

VAV box which is pressure dependant(no pressure control)

In environments where fluctuations in air volume is not critical

Variable air volume control-contd

VAV box which is pressure independant(with pressure control). Flow

controller controls the minimum flow requirement

In systems where a min qty of supply air must be provided under all conditions


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Supply air temperature reset control

The purpose of supply air reset temperature control is to reset the

supply air temperature when there is a reduced demand forcooling.

Chilled water temp reset control

Cooling Tower fan speed control

One of the most overlooked energy conservation strategies for

HVAC systems.

Control strategy: When demand

for condenser-water temperatureis reduced as a result of reduced

cooling load, modulate the speed

of the tower fan to maintain the

condensor water set point

Add Wet bulb temp

sensor/Humidity sensor

Lowering condenser water temp

allows chiller to operate more

efficiently


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Variable pumping strategies for

chilled water systemSingle Chiller with three way valves at load.

Most large chilled water system

have a diversity of 50% to 60%.

This contributes heavily to the

energy wasted in recirculating

unused water.

In fact, in commercial buildings

it is very common to find many

hours of low system loads while

mechanical plants continue to

serve the building at peak

capacity.


chilled water systemSingle Chiller with three way valves at load and VFD.

Though VFD saves energy by

minimizing the unused

circulating water

However the less water to chiller

can create problems. Coefficient

of performance can be

drastically altered


chilled water systemSingle Chiller with two way valves at load and By pass circuit

Constant chiller flow, variable system flow

Problems of low water flow to

chiller is solvedNO VFD, therefore no saving at

CHWP

Saving by chiller staging


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chilled water systemMultiple Chiller configuration- Constant chiller flow, Varaible

system flow

Addition of a temp sensor

The relationship between return

temp and supply temp indicates a

surplus or deficit of water supply

compared with water demand.

Measuring the system load by

valve postions.

Good for building loads closer

to the central plant, so the

distribution pumping power is

not a matter of great concern.


chilled water systemMultiple Chiller configuration-Primary and secondary chilled water supply loops

Chiller may operate independently under

constant flow

Distribution system will operate for the

building demand with variable flow

Flow switch to indicate the direction of flow

Flow meter measure the flow

Chiller plant operate under highly efficient

constant flow variable temp

Building load to operate in a varaible flow

constant temp which is most efficient inpumping capacity

Direct Digital control of air handling

systemVariable air volume system

Minimum outside air control, supply reset control, duct static pressure control,

and zone terminal control all act in concert to provide right quantity and

temeperature of air to the occupied space.


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Control of an HVAC system

Unit level control

Unit level control

Primary function is to protect

the equipment

Standalone-capable of

performing even if no support

from system level


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System Level control

Provides intelligent , cordinated

control

System Design Methodology

END

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HVAC Controls 2003