CHAPTER 7.

AIR-HANDLING EQUIPMENT AND SYSTEMS

7.1 Air-Handling Equipment

7.2 Heat Transfer

7.3 Air Cleaning

7.4 Air Mixing

7.5 Fans

7.6 Duct Systems

7.7 Air Devices

7.8 General Guidelines for Duct System Design

7.9 Under-Floor Air Systems

Components of air-handling units:

- Fan section(s)

- Heat-exchange section

- Humidification section

- Filter section

- Air-mixing section

- Discharge air plenum

7.1 Air-Handling Equipment

7.2 Heat Transfer

7.2.1 Water Coils

The construction of a typical water coil

The water coils are normally constructed of copper tubes and aluminum fins. Drain pans are required under cooling coils to collect condensate.

Heat transfer occurs at the heat-exchange section of the air handling unit. Commonly used heating and cooling media include water, steam, refrigerant, and

electric.

The performance of a heating or cooling coil depends on the design of its tubes and fins, as well as the size of the coil, including the depth and face area.

The depth of the coils is expressed in rows, which represent layers of tubes that conduct the heating or cooling fluid.

Tubes of adjacent rows are staggered to gain more contact between air and coil.

Depths of heating coils and cooling coils:

- Heating coils: 1 to 4 rows (because of high temperature difference between the heating fluid and the heated air)

- Cooling coils: 4 to 8 rows (because of low temperature difference between the cooling fluid and the cooled air)

Face velocities of heating coils and cooling coils:

-Heating coils: up to 6m/s

- Cooling coils: below 3m/s (to avoid carryover of condensate)

In steam coils, the tubes are designed for easy drainage of the condensate.

Tow types of steam coil:

- Conventional type (single-tube design): supply(steam) and return(condensate) at different ends of the coil.

- Steam-distribution type (tube-in-tube design): steam is distributed evenly from an inner orifice tube within the outer heat-transfer tube.

7.2.2 Steam Coils

Electrical coils may be designed as a part of the air-handling unit or installed on the ductwork exterior to the air-handling unit.

The heating elements are usually made of a nickel-chromium alloy.

Electric coils have very low resistance to airflow, so higher velocities can be used than with water or steam coils.

7.2.3 Electrical Coils

When the cooling medium is a refrigerant, the cooling coil is designed to allow the refrigerant to vaporize in the coil.

A typical DX coil consists of a refrigerant header and many distribution tubes.

7.2.4 Direct Expansion (DX) Coils

Header

Refrigerantinlets

Refrigerantoutlets

7.3 Air Cleaning

ParticulateNormal Size(µm) Particulate

Normal Size(µm)

Fumes 0.001-1 Tobacco smoke 0.01-1

Smog 0.001-2 Bacteria 0.3-30

Dust 0.001-20 Pollen 10-100

Viruses 0.003-0.05 Human hair 40-200

Common suspended particulates in urban air

Source : ASHRAE Handbook of Fundamentals, 1993.

Air in urban environment contains many impurities, in the form of gas, liquid, and solid particulates.

Many of these particulates are classified as pollutants, such as smog, smoke, and pollen.

In addition, air may contain bacteria and viruses.

7.3.1 Means of Cleaning Air

Air can be cleaned by passing it through a liquid curtain or spray or through a dry filter medium.

A liquid curtain or spray may use water or chemical solutions to remove the air particulates, but these solutions usually serve other functions, such as cooling or humidification.

The dry type of filtration is by far the most commonly used method for cleaning air.

Classification criteria of air filters:

- Filtration principle: Filtration by the medium or by electrostatic precipitation- Impingement: Dry medium or viscous medium- Configuration: Flat or extended surface (pockets, V-shaped or radial pleats)- Service life: One-time disposable or renewable- Performance: Low and medium efficiency, high efficiency particulate air

(HEPA), or ultrahigh efficiency (UEPA)- Special features: Order absorption, disposal of radioactive material, etc.

7.3.2 Typical Air Filters

In electrostatic filters, dust and fumes first are positively charged at 14,000 V and then enter a second electric field, where they are pushed by positively charged plates and attracted to the collector plates charged negatively.

For odor removal, adsorption-type filters are used to remove gaseous contaminants from the airstream.

These filters rely on extremely porous activated charcoal to collect the contaminant.

For residential and commercial buildings:

- Low-efficiency and medium-efficiency filters are adequate.- Bag-type or pleated filters are used for higher efficiency.

For health care facilities and laboratories:

- The filtration requirements are often dictated by codes and regulations.- HEPA filters are used for clean rooms and special laboratories to maintain very clean

environments or to remove hazardous particles.- HEPA filters are expensive and exhibit a high resistance to airflow, so their use is

limited to these special applications.

For hazardous materials:

- The filter housing is designed to pull the contaminated filter directly into a plastic bag so that the filter can be replaced without exposing it to the environment. This feature is called “bag out.”

7.3.3 Application of Air Filters

Outside air required for a building is usually ducted to the inlet of an air-handling unit by mixing with the return air.

The two airstreams(OA, RA) must be balanced with dampers to introduce sufficient outside air for ventilation, but not so much as to require excessive energy for conditioning during extremes of weather.

Typical air-mixing box

7.4 Air Mixing

Introducing large quantities of outside air for ventilation or free cooling will tend to over-pressurize a building.

Therefore provisions are made for relieving certain portion of the return air to outside.

The choice between a return air fan and a relief air fan depends on the relative resistance of the air paths.

A long relief air path that causes a high resistance to airflow would favor the use of a relief air fan.

Otherwise, a return air fan is sufficient.

A fan moves air used in HVAC systems to ventilate or transport heating or cooling.

All fans have a rotating impeller with blades; this increases the kinetic energy of air by changing its velocity. The increased velocity is then converted to pressure.

Two basic fan designs: centrifugal and axial.

7.5 Fans

7.5.1 Application of Fans

Centrifugal fans are generally used for air-handling application.

Axial fans have the advantage of being compact when installed in line with ductwork.

Outlet dampers are common for small units.

Inlet guide vane dampers are more energy-efficient and are used on larger air-handling units

Variable-frequency motor speed controllers are the most energy-efficient and have become common in recent years.

7.5.2 Controls of Air Volume (Flow Rate)

Motors can be placed within the fan cabinet or air-handling unit or can be mounted externally.

Fan wheels can be coupled directly to the motor or can be driven by belts and pulleys (sheaves).

Direct-drive fans operates at the same speed as the motor and are not adjustable.

Sheaves and belts allow fans to be designed for slower, quieter operation, and selection of pulley diameter.

7.5.3 Fan Drives

- Volume of air delivered per unit time (airflow rate): m3/s (cfm)

- Pressure created (static, velocity, and total pressures): Pa (inches of water column)

- Power input: W (horsepower, hp)

- Mechanical and static efficiency: percentage

- Other factors, such as sound level in noise criteria(NC) or decibels(dB), etc

7.5.4 Fan Performance and Fan Laws

The performance of a fan is measured by the following characteristics:

The most common procedures for developing the characteristics of a fan:

- The performance of a fan is tested from tested from shutoff conditions to nearly free deliveryconditions.

- At shutoff, the duct is completely blanked off; at free delivery, the outlet resistance is reduced to zero.

- Between these two conditions, various flow restrictions are placed on the end of the duct to simulate various conditions on the fan. Sufficient points are obtained to define the curve between shutoff and free delivery conditions.

- Pitot tube traverses of the test duct are performed with the fan operating at constant speed. The point of rating may be any point on the fan performance curve.

- For each case, the specific point on the curve must be defined by referring to the flow rate and the corresponding total pressure.

The fan laws in the previous table relate the performance variables for any dynamically similar series of fans. The variables are fan size D; rotational speed N; gas density r; volume flow rate Q; pressure ptf or psf ; power W; and mechanical efficiency ηt.

Fan Law 1 shows the effect of changing size, speed, or density on volume flow rate, pressure, and power level.

Fan Law 2 shows the effect of changing size, pressure, or density on volume flow rate, speed, and power.

Fan Law 3 shows the effect of changing size, volume flow rate, or density on speed, pressure, and power.

The fan laws apply only to a series of aerodynamically similar fans at the same point of rating on the performance curve.

They can be used to predict the performance of any fan when test data are available for any fan of the same series.

An example of the application of the fan laws for a change in fan speed N for a specific size fan.

For example, point E (N1 = 650) is computed from point D (N2 = 600) as follows:

At D,

Q2 = 3 m3/s and = 228 Pa

Using Fan Law 1a at point E,

Q1 = 3×650/600 = 3.25 m3/s

Using Fan Law 1b,

Ptf1= 228(650/600)2 = 268 Pa

The total pressure curve at N = 650 may be generated by computing additional points from data on the base curve, such as point G from point F.

If equivalent points of rating are joined, as shown by the dashed lines, they form parabolas, which are defined by the relationship:

The line is called SYSTEM LINE.

7.5.5 Examples of Fan Performance

Given conditions:

1) An air-handling system is designed to circulate 15,000 cfm at a system pressure 2.7 in. w.c.

2) A fan is selected on the basis of the previous graph (Barry Blower Model Versacon).

Problems:

1) With this particular choice of fan, what should be the fan speed?

2) What is the Brake Horsepower (BHP) of the electric motor to drive the fan operating at this condition?

3) If the same fan is operating at 825 rpm, what will be the fan delivery, the air-handling system static pressure, and the BHP required?

BHP (Brake Horsepower) is the amount of power generated by a motor without considering any of the various auxiliary components that may slow down the actual speed of the motors. Sometimes referred to as pure horsepower, BHP is measured within the engine’s output shaft.

Units used in air-conditioning engineering

1 hp = 0.7457 kW (approximately 0.75 kW)

Answers:

1) From the graph, the intersection of 15,000 cfm with the system curve demands that the fan run at 1125 rpm.

2) The fan should be driven by a motor having a minimum of 8.55 BHP.

3) At 825 rpm, the fan can deliver 11,000 cfm, operating at 1.46 in. w.c., and drawing 3.39 BHP.

7.6 Duct Systems

Ductwork is part of the air-handling system. Duct system includes ducts for supply air, return air, outside air, relief air, and exhaust

air. Ducts are usually fabricated from sheet metal, such as galvanized steel, aluminum, or

stainless steel. Some ducts are made with nonmetals, such as plastics.

7.6.1 General Classifications for Ductwork

Ductwork systems for HVAC are classified by static pressure.

Recommended air velocities:

7.6.2 Symbols for Sheet Metal Work

There are various standard symbols for sheet metal work to be used in drawings.

One of the commonly accepted standards is the SMACNA Symbols.

Rectangular ducts

- Mostly used for low-velocity applications.- Insulation is often applied to the interior of the duct for both acoustical absorption

and thermal insulation.- The insulation applied internally is called duct liner. - In cooling applications, liner also prevents condensation on the outer surface of

the duct.

7.6.3 Duct Shapes and Insulation Methods

Round or flat oval ducts:

- Can be used for low-velocity, but these shapes are generally reserved for medium-and high-velocity ductwork.

- If insulation is required, it is usually applied externally in the form of a fiberglass blanket wrap with an external vapor barrier for cooling applications.

- The vapor barrier is intended to prevent the migration of humid air through the insulation, which could result in condensation on the surface of a cold duct.

- Internal insulation is also available for round or flat oval ductwork.

7.6.4 Duct Shapes and Insulation Methods

Galvanized steel is the most widely used material.

Aluminum, stainless steel, and plastic may be used for ducts installed in humid environments or ducts that carry moist air, such as a dishwasher exhaust.

Fire-resistant steel ductwork is used for kitchen hoods.

Assembly of high-pressure/velocity round ducts with low-pressure rectangular ducts

7.6.5 Materials of Construction

Round ducts have the most cross-sectional area per unit area of sheet metal and thus are most economical. When ceiling space is limited, oval ducts fit the need. Round or oval ducts are available in double-wall construction to reduce the air-

transmitted and duct-radiated noises. Ducts should be sealed in compliance with the recommended duct seal levels for

different applications. Exposed supply ductwork in conditioned spaces should be seal level A to prevent dirt

smudges. Responsibility for proper assembly and sealing belongs to the installing contractor.

Working on duct assembly

7.6.6 Duct Assembly and Air Leakage

Duct seal

Transverse joints

Longitudinal seams

Plastic film and fabric ductwork is good alternative to sheet metal duct work in high-humidity spaces such as swimming pools and greenhouses.

The rigidity and tubular shape of these ducts is achieved by internal air pressure, and air is diffused through holes (plastic duct) or through pores (fabric).

Plastic-film ductwork(greenhouse) Different applications of fabric ductwork

In most commercial buildings, ductwork shares space above the ceiling with other elements including structural supports, fireproofing, electrical conduits, sprinkler piping, and light fixtures.

Clearances must be provided between ductwork and the lighting fixtures below them to allow the fixtures to be removed (3 inches, approximately 8cm).

Congested ceiling space and the importance of coordination between systems

7.6.7 Coordination of Ductwork with Other Building Elements

Inner wall is constructed of perforated metal

7.6.8 Materials and Fittings for Sound Control

Sound attenuators, also called sound traps, are special duct fittings containing sound-absorbing material faced with perforated metal.

7.7 Air Devices

7.7.1 General Classifications

Air devices, used for supplying air and removing air from space, are among the few parts of the HVAC system that are visible to the occupants of a building.

Air devices include diffusers, grills and registers, flow control devices, and other accessories.

Grills simply contain vanes; registers contain a control damper behind the vanes.

Coanda Effect

Ceiling diffusers rely on a phenomenon called the Coanda effect, in which cold air clings to the ceiling upon being discharged from the diffuser.

The Coanda effect allows the airstream to fall gradually and mix with the room air over a large area.

If ceiling diffusers are used to deliver warm air, the warm air has a tendency to stay at the ceiling, owing to its low density.

The velocity must be sufficient to create turbulence and mixing with cold air.

The area that can be effectively served by a diffuser is affected by a parameter called throw.

If the velocity is too small, complaints of stuffiness will result.

If the velocity is too high, drafts will result.

7.7.2 Special Concerns for Warm-Air Supply

7.7.3 Spacing, Distribution, and Area of Coverage

Throw: The horizontal or vertical axial distance an airstream travels after leaving an air outlet before the maximum stream velocity is reduced to a specified terminal velocity (e.g., 0.25, 0.5, 0.75, or 1.0 m/s), defined by ASHRAE Standard 70. Room air velocities less than 0.25 m/s are generally preferred.

Air distribution pattern from ceiling air diffusers

A well-designed duct system will result in the lowest installation cost and most energy-efficient operation of the air-handling system.

Duct sizes are selected that will produce the highest possible velocity consistent with a reasonable friction loss.

Duct installations are most economical when maximum use is made of straight duct runs, and the number of fittings is minimized.

Duct systems that are not properly designed will create objectionable noise. Insulation placed inside the ductwork will attenuate noise radiating from the system.

Doors should be provided in the duct walls to allow access for cleaning inside the duct and for maintaining components in the ductwork.

Coordination of the ductwork with lighting layout and structural elements is an important criterion in the architectural design process.

7.8 General Guidelines for Duct System Design

Ductwork plan of a simple HVAC system showing supply air ducts and heating water piping

Ceiling grid and lighting fixtures are added for coordination between different space elements

7.9 Underfloor Air Systems

Delivering air from underfloor is an alternative to conventional overhead air distribution.

Underfloor distribution for commercial buildings generally uses 60x60cm access-floor panels supported by pedestals on the structure slab to form an air plenum.

Air is introduced to the space through floor-mounted diffusers which are specially designed with cleanable receptors for dust and debris.

Air is introduced to the space through floor-mounted diffusers.

Air velocities must be low, temperatures are generally 15℃ to 18℃

Floor diffuser on access floor with carpet tiles

Specially designed with cleanable receptors for dust and debris

Air-handling devices for underfloor HVAC system