HVAC Duct System

Air Conditioning systems use an air distribution network by ducts to circulate conditioned air to all the conditioned spaces in retail, office or residential buildings. To maintain uniform temperature throughout the space efficiently, the system should be designed properly.

Ducts can be classified by its applications and pressure.

Applications

Residential, Commercial and Industrial Applications

Pressure

Low Pressure        (Velocity ≤ 10m/s & Static Pressure ≤ 50mm Wg)

Medium Pressure    (Velocity ≤ 10m/s & Static Pressure ≤ 150mm Wg)

High Pressure        (Velocity ≥ 10m/s & Static Pressure 150mm ≤ Ps ≤ 250mm wg)

Air Velocities should be within the acceptable range to reduce noise.

Recommended air velocities mainly depend on the areas to be served and noise criteria.

Acceptable range of air velocities are

Residence Application     3m/sec to 5 m/sec

Theaters        4m/sec to 6.5 m/sec

Hotels            7.5m/sec to 10m/sec

Normally 8 to 10 m/sec is used for Main ducts flow and 4 to 6 m/sec is used in branch ducts. The maximum of 30 m/sec is used in ships and aircrafts to reduce the requirement of space.

Ducts are commonly fabricated from galvanized steel in rectangular and circular shapes. In air conditioned application, it will be insulated with insulation materials such as nitrile rubber, fibre glass etc. In external areas, insulation will be covered with aluminum cladding to prevent it from damage.

Galvanised sheet of various thicknesses has been used for duct construction. It depends on duct width. Recommended thicknesses are

Upto 300mm         0.55 to 0.7 mm (26G & 24 G)

300mm to 1500mm    0.85 mm (22 G)

1500mm to 2250mm    1.066mm (20 G)

2250mm and above    1.3mm (18 G)

In some applications, aluminium, stainless steel fabricated ducts will be used such as kitchen extraction ducts etc.

Now a day, polyurethane and phenolic panels are used as pre insulated ducts. 80 microns thickness is considered for internal use and 200 microns thickness is considered for some external use.

Refrigerants

A refrigerant is a fluid used in refrigeration system to transfer energy by its phase change from liquid to gas and vice versa. The refrigerants are used in Domestic Refrigerators, Air Conditioners and Central Chilling Plants. The best suitability of a refrigerant for a particular system is based on its thermodynamic properties, chemical properties, physical properties, safety and economic criteria.

The critical temperature should be as high as possible above the condensing temperature in order to have larger range of isothermal energy transfer. The specific heat of the fluid should be as small as possible. The thermal conductivity should be as large as possible so that size of evaporator and condenser becomes smaller. The Freezing point of refrigerant should be as low as possible.

The density of vapour refrigerant should be as large as possible. The refrigerant should be inert and should not react with the materials of refrigeration system. It should be chemically stable for the operating conditions. The refrigerant should be non toxic and non flammability in safety concern.

Here a few refrigerants which are in the common commercial use.

R11 – Trichlorofluromethane

R12 – Dichlorofluromethane

R13 – Chlorotrifluromethane

R14 – Tetrafluromethane

R21 – Dichlorofluromethane

R22- Chlorodifluromethane

R113 – Trichlorotrifluromethane

R123 – Dichlorotrifluroethane

R134a – Tetrafluroethane

As chemical names of refrigerants are long and complex, a method of referring refrigerants by numbers was formed. The method of designating a refrigerant by number as follows.

1^st Digit on the Right    - No. of Fluorine atoms

2^nd Digit from the Right    - No of Hydrogen atoms plus one

3^rd Digit from the Right    - No of Carbon atoms minus one (not used when equal to zero)

Example:

R 123 CHCl₂CF₂ Dichlorotrifluroethane

No of Fluorine atoms = 3

No of Hydrogen atoms + 1 = 2

No of Carbon atoms – 1 = 1

Thus it forms R 123

The lower case letter that follows the refrigeration designation refers to the form of the molecule when different forms (isomer) are possible. The number alone will be indicated if the form is symmetrical. If form becomes unsymmetrical, the letters a, b, and c are appended such as R134a.

The refrigerants can also be abbreviated as follows

Chloroflurocarbons (CFC)

Hydrochloroflurocarbons (HCFC)

Hydroflurocarbons (HFC)

Hydrocarbons (HC)

Refrigerant blends are formed by mixing two or more refrigerants. Blends can have as many as four refrigerants mixed together to give properties and efficiencies similar to the refrigerants. The blends are referred in 400 and 500 series in R Numbers. If a blend has R 407 designation, it is a near azeotropic blend. The 7 indicates that it is the seventh one produced in the series in the market.

ODP and GWP Is the most important terms used in refrigerants. ODP means Ozone Depletion Potential. Ozone is a gas layer found in stratosphere and troposphere to prevent harmful UV radiation from reaching the earth. Now it is rapidly being depleted by manmade chemicals containing chlorine, including refrigerants such as CFC and HCFC. An index called ODP is used for regulatory purposes. The higher the ODP, the more damaging chemicals to the ozone layers. The heat energy radiated by earth back into the atmosphere is absorped, reflected or refracted by gases such as CFC, HCFC and CO₂ and prevented from escaping to atmosphere. This warming process caused by atmospheric absorption is called Greenhouse effect or Global Warming. This is measured by an index Global Warming Potential (GWP). Refrigerants leaking from the system contributes to global warming. This is measured by comparing them to carbon dioxide which has a GWP of 1. CO₂ is the number one contributor to global warming.

There are many applications which restrict the direct use of refrigerants due to safety consideration. Under such circumstances the cheaper grade cooling media such as water, brine solution of sodium chloride or calcium chloride is selected.

ASHRAE Advanced Energy Design Guide

American Society of Heating Refrigeration Air Conditioning Engineers (ASHRAE) has given an opportunity to download its advanced energy design guides for small warehouse, office buildings, retail buildings and school buildings for free. 

To download it free you will need to register at http://www.ashrae.org/freeaedg

Air Conditioning & Refrigeration System

Refrigeration deals with the transfer of heat from a low temperature level at the heat source to a high temperature level at the heat sink by using a low boiling refrigerant.

Depending on applications, there are several types of equipments are available for use

as given below:

Air Conditioning

Window/Split/Package Air Conditioners

Fan Coil Units

Air Handling Units

Refrigeration Systems

Small capacity modular units of direct expansion type similar to domestic refrigerators,

small capacity refrigeration units.

Centralized chilled water plants with chilled water as a secondary coolant for temperature

range over 5°C typically.

Brine plants, which use brines as lower temperature, secondary coolant, for typically sub

zero temperature applications.

The plant capacities upto 50 TR are usually considered as small capacity, 50 – 250 TR as

medium capacity and over 250 TR as large capacity units.

A large industry or High Rise Tower Buildings may have a bank of such units with common chilled water pumps, condenser water pumps, cooling towers, as an off site utility.

The same industry may also have two or three levels of refrigeration & air conditioning such as:

 Comfort air conditioning (20° – 25° C)

 Chilled water system (8° – 10° C)

 Brine system (Sub-zero Applications)

Two principle types of refrigeration plants found in industrial uses are: 

Vapour Compression Refrigeration

Vapour Absorption Refrigeration

Vapour Compression System uses mechanical energy as the driving force for refrigeration, while Vapour Absorption uses thermal energy as the driving force for refrigeration.

HVAC Introduction

HVAC is an acronym that stands for Heating Ventilating Air Conditioning. HVAC is for Controlling temperature and humidity to bring comfort to occupants inside the certain spaces such as Office Buildings, Residential Towers etc.

 Air Conditioning refers the cooling and dehumidification of indoor air for thermal comfort. Air Conditioning is based on the principles of Thermo dynamics, Fluid Mechanics and Heat Transfer.

 Refrigeration Cycle is more important for Air Conditioning System. 

The dome-like curve represents saturated conditions for the refrigerant. On the left half of the dome, the refrigerant exists as a saturated liquid and on the right as saturated vapor. Both liquid and gaseous refrigerant coexist inside the dome in saturation. To the left of the dome, the refrigerant is a sub cooled liquid and to the right of the dome, it is a superheated vapor.

The numbers (1 through 4) represent significant points in the flow of refrigerant as it makes its circuit in the cycle. The refrigerant working fluid undergoes thermodynamic changes between these points.

Point 1-2 (Evaporation): Since this is inside the d

ome, constant pressure (21.5 psia) and temperature (-5°F) are maintained, i.e., saturation. When heat is transferred at saturation, the result is a change in phase.

Point 2-3 (Compression): Compressing the gaseous Freon from 21.5 to 141 psia (6.5 to 126 psig) produces a concomitant increase in thermal energy represented by a rise in the enthalpy and the temperature of the Freon from 5° to 125°F. This is the heat of compression resulting from the added energy to the Freon vapor. Compression provides the thermal driving head to sustain the flow of Freon through the cycle.

Point 3-4 (Condensation): In passing through the dome from the right side to the left, the refrigerant cools from 125° to 105°F and changes phase from a superheated vapor to a slightly subcooled liquid.

Point 4-1 (Expansion): The refrigerant is expanded by passing through an expansion valve where its pressure is reduced from 141 psia to 21.5 psia. In the process of expanding, the Freon cools from 105° to -5°F (cold of expansion) and crosses into the dome where both saturated liquid and gaseous Freon can coexist. About 25% of the fluid vaporizes into a gas during the process.

The typical Refrigeration Cycle would be as follows

The following are the Components of Refrigeration System.

Evaporator/Chiller

Compressor

Condenser

Receiver

Thermostatic Expansion Valve (TXV)

The refrigerant will be used in the refrigeration system. 

Evaporaor to be located in the space to be refirgerated or cooled.

Compressor is used to compress the refrigerant to achieve heat transfer.

Condenser in which refrigerant rejects its heat to cooling medium either air or water.

Expansion Valve is used to bring down the pressure of refrigerant before entering into evaporator.