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Johnson Controls Inc, Heating, Ventilation and Air Conditioning (HVAC) Controls Variable Air Volume (VAV) Systems, VisSim Tutorial, Part I

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Johnson Controls Inc, Heating, Ventilation and Air Conditioning (HVAC) Controls Variable Air Volume (VAV) Systems, VisSim Tutorial, Part I

; VisSim Tutorial

    ; Heating, Ventilation and Air Conditioning (HVAC) Controls:

    Variable Air Volume (VAV) Systems

Nebil Ben-Aissa, Johnson Controls, Inc.

    ; Introduction

    The purpose of Heating, Ventilating and Air Conditioning (HVAC) control systems is to keep people comfortable within an enclosed space. Although comfort control is thought of as achieving a desired temperature in a building, it is also achieved by maintaining a desired level of humidity, pressure, radiant energy, air motion and air quality.

    One of the most popular HVAC applications, the Variable Air Volume (VAV) application, is designed to deliver low energy cost, low maintenance and good comfort performance. The VAV application controls the temperature inside a space by modulating the amount of air supplied to it. VAV systems, which are studied in this chapter, are easy to install, commission and service. They are also simple to understand and represent a good introduction to HVAC controls.

    In this tutorial, the VAV system is analyzed, modeled and simulated. The air handling system, which provides air to the VAV system, is also studied and analyzed. When modeling the different components of the VAV system, practical models are used to capture the different characteristics of the system. The VAV system components used in this tutorial are fairly simplified using first order system modeling and piece-wise linear modeling techniques. The different models used to simulate the VAV system response are easy to develop and are great for engineers who are interested in observing the general behavior of the system rather than the minute details.

    All of the models used in this tutorial are derived using English IP (Inch Pound) units instead of metric SI (Systéme International) units. Table 1 shows the IP unit abbreviations used in this tutorial. Table 1. IP Unit Abbreviations

    Abbreviation Definition

    Btu British Thermal Units

    Cfm Cubic Feet per Minute

    ?F Degrees Fahrenheit

    ft Feet

    in Inches

    lb Pounds

    sqft Square Feet

     Copyright 1997. Visual Solutions, Inc. All rights reserved. Author: Nebil Ben-Aissa, Johnson Controls, Inc.

     VisSim Tutorial 2;

    ; The VAV Air Handling System

    The air handling system is the primary HVAC system in most building HVAC installations. This system, which is hidden from most building occupants, is the main system that delivers conditioned air to the entire building. The main responsibility of the air handling system is to supply the building with fresh and conditioned air, then exhaust it from contaminated and carbon dioxide (CO) air. Most air handling 2

    equipment is located on building rooftops. The size and configuration of the air handling unit, also referred to as the rooftop unit, depends on the size and requirements of the building HVAC specifications.

Figure 1. Air handling system

    As shown in Figure 1, the air handling system delivers heated or cooled supply air to the multiple VAV systems attached to the building’s air ducts. Since the VAV system control is dependent on the operation of the air handling system, it would be easier to understand the sequence of operation of the air handling system before analyzing and modeling the VAV control system.

    The air handling unit consists of two main components:

    ; The air handling equipment, which consists of outside air dampers, mixed air dampers, heating

    coils, cooling coils, and supply air fans.

    ; The air handling controls, which consists of the electronic control hardware that sequences the

    operation of the air handling system.

    The air handling control operation, as shown in Figure 1, is described by the following four steps:

    1. Mix outdoor air with return air

    2. Control the supply air pressure

    3. Heat/cool the mixed air

    4. Control the exhaust air pressure

    ; The VAV System

    The VAV control concept was derived from the realization that most of the air conditioning operation in a building is cooling only. Independently of the outdoor air climate conditions, most occupied rooms need cooling only to eliminate the energy loads caused by solar radiation, human occupancy, equipment and utility operation.

    The operation of the VAV cooling-only control system, assuming that there is enough air pressure in the duct system and that the temperature of the supply air is cold enough, is described by the following steps:

    1. The VAV controller monitors the temperature in the room.

     Copyright 1997. Visual Solutions, Inc. All rights reserved. Author: Nebil Ben-Aissa, Johnson Controls, Inc.

    Heating, Ventilation and Air Conditioning (HVAC) Controls: Variable Air Volume (VAV) Systems 3;

    2. If the room is warm, the VAV controller opens the supply air damper to allow more cold air into

    the room.

    3. If the room is cold, the VAV controller closes the supply air damper, thus allowing the interior

    loads to heat up the room space.

    4. If the room is occupied, the VAV supply air damper cannot be fully shut. The VAV controller has to

    maintain a minimum amount of fresh air, specified by the building designers, so that people

    occupying the room do not suffocate.

    This cooling-only VAV control application, more specifically defined as the “single-duct, pressure-

    dependent, cooling-only” VAV application and shown in Figure 2, will be the main focus of the rest of this tutorial.

Figure 2. Single-duct, pressure-dependent, cooling-only VAV system

    Like all control systems, this VAV system has three main components:

    ; The VAV process

    ; The VAV control

    ; Sensors and feedback components

    Figure 3 describes the different control, process and feedback components of the VAV system.

Figure 3. VAV system components

    ; The VAV Process

    The VAV process contains the following components:

    ; The room

    ; The VAV damper

     Copyright 1997. Visual Solutions, Inc. All rights reserved. Author: Nebil Ben-Aissa, Johnson Controls, Inc.

     VisSim Tutorial 4;

    ; The Room

    Let’s consider the case when the room is warm. As the temperature of the room increases, the VAV controller opens the damper to allow in more cold air. The heat generated by the internal and external loads are dissipated by mixing the room’s warm air volume with the cold supply air volume.

    The process of mixing and exchanging energy between air volumes is governed by the laws of thermodynamics. These laws describe the different behaviors of energy in gases, liquids and solids. The first law of thermodynamics, referred to as the law of conservation of energy, states that the sum of all energies entering and leaving an enclosed space is equal to the rate of change of stored energy in the same space.

    The equation describing the first law of thermodynamics is

     (1)

    where

    Qin is the energy entering the room space

    Qout is the energy leaving the space

    d(Q)/dt is the rate of change of the stored energy

    The energy Q of a gas is defined by the following equation:

     (2)

    where

    M is the mass of the gas. M of the gas is also defined as

    Cp is the specific heat constant

    is the room temperature differential. It is also equal to

     Copyright 1997. Visual Solutions, Inc. All rights reserved. Author: Nebil Ben-Aissa, Johnson Controls, Inc.

    Heating, Ventilation and Air Conditioning (HVAC) Controls: Variable Air Volume (VAV) Systems 5; Combining equations 1 and 2 and applying the resulting equation on the process of mixing the room’s air

    volume with the supply air volume, the following equation 3, is obtained:

     (3)

    where

    , Rho air, is the density constant of air

    V is the volume of the room room

    T is the temperature of the room room

    Q is the energy generated by interior loads, such as people, lights, and computers interior

    Q is the energy generated by exterior loads, such as the sun and cold weather exterior

    Since

and

    T = T roomair leaving room

    then

Therefore

    ,

     Copyright 1997. Visual Solutions, Inc. All rights reserved. Author: Nebil Ben-Aissa, Johnson Controls, Inc.

     VisSim Tutorial 6;

     (4)

    where

    Volume is the volume of the air flow supplied into the room SupplyAirFlow

    K is a constant

    T is the temperature of supplied air into the room supply

    Combining equations 3 and 4 yields

     (5)

    To establish an equation that determines the room temperature, rewrite equation 5 as follows:

     (6)

    Replacing the following constants with their value (in IP units), the following final equation is obtained:

     (7)

    where

    Cp = 0.241 Btu/(lb ?F)

    3 = 0.075 lb/ft

    K = 1.08 (Cfm Btu)/(Hour ?F)

    The VisSim simulation model of the room is described in Figure 4.

     Copyright 1997. Visual Solutions, Inc. All rights reserved. Author: Nebil Ben-Aissa, Johnson Controls, Inc.

    Heating, Ventilation and Air Conditioning (HVAC) Controls: Variable Air Volume (VAV) Systems 7;

Figure 4. VisSim implementation of the room model

    ; The VAV Damper

    The VAV damper is a mechanical device that controls the amount of supply air flow discharged into the room. Although there are many damper designs available in today’s HVAC industry, the most common damper design in VAV applications is a single-axle, circular, thin metal damper rotated by an actuator, as shown in Figure 5.

Figure 5. The VAV damper

    The amount of air supplied by the damper into the room depends on the angle at which the damper is positioned. As can be seen in Figure 5, when the damper is at 90?, or 0% open, the damper is fully shut and no air is supplied to the room. On the other hand, when the damper is at 0?, or 100% open, the damper supplies the maximum amount of air allowed by the duct pressure. Therefore, by observing the amount of flow supplied by the damper versus the damper position, the damper characteristics can easily be seen as illustrated in Figure 6.

Figure 6. Damper characteristics

    From Figure 6, it is concluded that there is a nonlinear relationship between the amount of air supplied and the damper’s position. In fact, laboratory experiments have shown that for most commercial VAV dampers, when the damper position is approximately between 0% to 15%, the amount of air supplied by the damper is pretty low. The same experiments reveal that when the damper position crosses the 15% to 70% range, the air flow suddenly becomes more significant and small position changes cause high air flow gains. Finally, when the damper position is approximately between 70% to 100%, low air flow gains are observed. This is because most of the air flow has been already delivered by the time the damper position reaches the 60% to 70% range, and additional angular increments of the damper cause low air flow gains.

     Copyright 1997. Visual Solutions, Inc. All rights reserved. Author: Nebil Ben-Aissa, Johnson Controls, Inc.

     VisSim Tutorial 8;

    The nonlinear characteristics of the VAV damper are usually modeled with a third or higher order polynomial. These models can get a lot more complicated depending on the type of damper and the level of sophistication required from the model. However, since we are trying to capture the general dynamics of the damper characteristics, a simplified piece-wise linear model can be satisfactory. To model the damper characteristics using a piece-wise linear model, the following algorithm is used: Begin Algorithm A-1

    If (Damper Position <= 18%) Then

     a = 0.5, b = 0

    Else If (Damper Position >= 18%) AND (Damper Position <= 68%) Then

     a = 1.66, b = -20.88

    Else If (Damper Position >= 68%) Then

     a = 0.25, b = 75.0

    Supply Air Flow = (a * (Damper Position) + b ) * (Maximum Air Flow /100)

    End Algorithm A-1

    Figure 7 shows the VisSim implementation of the piece-wise linear model described by algorithm A-1.

Figure 7. Piece-wise linear damper model described by algorithm A-1

    A VisSim plot of the response of the damper model is shown in Figure 8.

Figure 8. Piece-wise linear damper characteristics

    Note the similarities between Figure 6 and Figure 8, and how the piece-wise linear model was able to approximate the characteristics of the damper response without using a complicated third or higher order polynomial model.

    The damper can also be sized using the room equations developed earlier. Using equation 4, the maximum air flow volume which eliminates the different internal and external room loads, is described as follows:

     Copyright 1997. Visual Solutions, Inc. All rights reserved. Author: Nebil Ben-Aissa, Johnson Controls, Inc.

    Heating, Ventilation and Air Conditioning (HVAC) Controls: Variable Air Volume (VAV) Systems 9;

     (4)

    Therefore

     (8)

    where

    MaxVolume is the maximum air flow per minute delivered by the damper SupplyAirFlow

    Q is the supply air load, also referred to as the sensible load of the room supply

    T and Tare the temperatures of the room and the supply air, respectively roomsupply

    ; The VAV Control

    Most VAV controllers are mounted on the VAV box hardware. The embedded VAV controller, which

    usually contains the control electronics and the actuator, mounts directly on the VAV damper actuator

    axle, as shown in Figure 9.

Figure 9. Installed VAV box with controller

     Copyright 1997. Visual Solutions, Inc. All rights reserved. Author: Nebil Ben-Aissa, Johnson Controls, Inc.

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