TAHLEEQ: GRADUATE TRAINING PROGRAM

TECHNICAL REPORT 1

“HVAC SYSTEMS”

WORK PLACEMENT 1

PLACEMENT DEPARTMENT (OR CONTRACTOR): ETEC (MECHANICAL)

PLACEMENT CITY/LOCATION: BEIRUT, LEBANON

PREPARED BY: HUSAIN ALAWI S SHUBBER BAQER

7th NOV 2014

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Table of Contents

1. Summary ................................................................................................................................................................................................... 3

2. Scope of Work .......................................................................................................................................................................................... 3

3. HVAC Systems .......................................................................................................................................................................................... 3

3.1 Cooling System .................................................................................................................................................................................. 4

3.1.1 Chiller .......................................................................................................................................................................................... 4

3.1.2 Cooling Tower ............................................................................................................................................................................. 4

3.1.2.1 Cooling Tower Construction ................................................................................................................................................ 5

3.1.2.2 Cooling Tower Process ....................................................................................................................................................... 5

3.1.3 Condenser Pump ........................................................................................................................................................................ 6

3.1.3.1 Traditional Water Pumping System ..................................................................................................................................... 6

3.1.4 Primary Pump ............................................................................................................................................................................ 7

3.1.5 Secondary Pump ........................................................................................................................................................................ 9

3.1.5.1 Traditional Secondary Pumping System ............................................................................................................................. 9

3.2 Air Handling Unit (AHU) For Clean Room System ............................................................................................................................. 10

3.2.1 Filters ........................................................................................................................................................................................ 10

3.2.2 Air Intake Fan ........................................................................................................................................................................... 10

3.2.3 Cooling and Heating Coil ........................................................................................................................................................... 10

3.3.4 Exhaust Fan ............................................................................................................................................................................... 11

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1. Summary The purpose of this technical report is to discuss some of the technical aspects involved in the

mechanical design of the HVAC systems. Figures are illustrated in order show the key aspects in each

HVAC system and how it works in conjunction with other related systems. This report was prepared for

my own reference and as part of my learning process during the work placement.

2. Scope of Work All the technical aspects discussed will be limited to the design, application, methods for operating and

control, and services of HVAC systems in the Central Utility Complex (CUC). The HVAC systems at

Bahrain Airport are limited to Cooling and Air Handling Unit (AHU).

3. HVAC Systems Is the technology of indoor and vehicular environmental comfort. The main goal of this system is to

provide thermal comfort and acceptable air quality. However, there three main sections involved in this

system, one is the cooling system, two is the heating system (which is not required for The Passenger

Terminal Building PTB at Bahrain Airport), and three is the air handling unit (AHU). All the three systems

are illustrated in the figure below (See Figure 1) showing how they are connected in conjunction with

each other.

Figure 1: Shows the three sections of HVAC system and how they are connected together

Air Handling Unit (AHU)

Heating System

Cooling System

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3.1 Cooling System The cooling system uses the temperature of the environment that the HVAC is controlling. There are five

main parts in this system that are:

1. Chiller

2. Cooling Tower

3. Condenser Pump

4. Primary Pump

5. Secondary Pump

3.1.1 Chiller

As far as Bahrain Airport’s PTB is concerned, it has been decided to use the Air-Cooled Chiller System.

ETEC has provided as study comparing two main chiller systems (Water-Cooled Chiller & Air-Cooled

Chiller Systems). Conventional thinking promotes water-cooled chillers as a better choice in terms of

efficiency. Looking only at the compressor costs, this may be true. However, using state-of-the-art

technology with centrifugal compressor and variable control, air-cooled chillers are often the better

choice.

It’s important to look at the total operating costs involved, not only the compressor cost. In water-

cooled chiller system, cooling tower’s operating cost should be added to the total operating cost of the

water cooled chiller system. Cooling tower cost include the tower fan, water and sewer costs, chemical

costs (as the water should be chemically treated), and pumping cost.

Listed below, are the three main points concluded from the comparison study that was performed by

ETEC:

Initial cost: air cooled system (B.D. 3,310,000.00) less than water cooled

(B.D. 4,405,000.00).

Total yearly operating cost: air cooled system (B.D. 734,815.00) less than water cooled (B.D.

823,610.00).

Space Allocation: air cooled system (3012 m2) less than water cooled (4834 m2).

Hence, ETEC has recommended using Air-Cooled Chillers for the production of chilled water.

(Note: This information is taken from the Comparison Report which has been performed by ETEC)

3.1.2 Cooling Tower

Cooling towers are used to cool warm condenser water in Water Cooled Chiller System (The use of

cooling tower for the chiller system recommended for PTB is not applicable, as Air Cooled Chiller will be

used). The warm water absorbs heat from the chiller’s condenser section and releases it into the

atmosphere in the cooling tower. The tower fans provide moving air to cool the warm water primarily

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be evaporation (i.e. evaporative cooling). The concept of this process is to return a constant condenser

water temperature to the chiller.

3.1.2.1 Cooling Tower Construction

The three cell cooling tower shown in figure 2 has axial fans mounted on the top of the tower. These

fans draw cooling air through the sides of the tower and exhaust through the top of the tower.

The shafts of the motors are horizontal while the fan shafts are vertical – See Figure 2 below. A right

angle gearbox is mounted at the base of each fan and along drive shaft connects the drive motor to the

gearbox.

Figure 2: Shows the construction of the cooling tower

3.1.2.2 Cooling Tower Process

Now after explaining the construction of the cooling tower, let’s have look at how the cooling process

works. The warm water is sent through a water inlet pipe to the cooling tower. After that, it’s

distributed across the top of the cooling tower and gravity causes the water to cascade down the sides

across fill material – See Figure 3.

The fill material causes the water drops to break into smaller water droplets, which results in creating a

larger area for heat transfer. The cooling tower draws air across the water droplets, cooling by an

exchange of latent heat resulting from evaporation of a small amount of water. After passing through

the cooling tower, the water collects in the tower basin and is pumped back to the chiller condenser

section by the condenser water pump – See Figure 3. It should be noted that in cold weather operation,

the cooling tower will have a heater in the tower basin to keep the water from freezing.

Cooling tower fans need to be controlled because cooling towers are designed for maximum load

conditions. However, in most of the applications, maximum load occurs for only less than 10% of the

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time. Therefore, if controlling flow becomes important, the speed of the motor driving the cooling tower

fan can be controlled.

Controlling the motor:

Saves energy

Reduces wear and tear

Reduces audible noise

Reduces water use

Figure 3: Shows the process used to cool warm condenser water “Evaporative Cooling” in cooling tower

3.1.3 Condenser Pump

Condenser water pumps are primarily used to circulate water through the condenser section of the

water cooled chillers and their associated cooling tower. The condenser water absorbs the heat from

the chillers condenser section and releases it into the atmosphere in the cooling tower by evaporation.

The cooled water collects in the tower basin where it is then pumped back to the chiller condenser.

3.1.3.1 Traditional Water Pumping System

Traditionally, a condenser water pump continuously circulates the water at full flow. The temperature if

the condenser water is determined by the cooling tower fans operation:

The lower the condenser water temperature the lower the energy consumption by the chiller .

WATER INLET PIPE WATER INLET PIPE

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However, the decrease in energy consumption is small and reducing the speed of the cooling tower fans

can save more energy. The condenser water pump continuously circulates the cooling tower water at a

constant flow. The chiller manufacturer recommends the flow through the condenser section for best

heat rejection. Often the condenser pump is oversized for a safety margin and requires field balancing

with a balancing valve to increase the system resistance and to prevent too high flow rate, as excess

flow can erode the chiller tubes which results in increasing maintenance.

3.1.4 Primary Pump

Boilers and chillers (only boilers will be used for Bahrain Airport HVAC system) are constant flow devices

in which constant flow of water is necessary for efficient and safe operation. In order to provide

constant flow, HVAC systems are designed with two independent water loops that are:

1) Primary Loop

2) Secondary Loop

Primary Loop primary loop provides constant flow through the boiler or chiller, and the Secondary Loop

provides variable water flow throughout the buildings to air handling units or other devices with heating

or cooling coil.

Primary pumps in primary/secondary systems have the task of supplying a constant flow of water

through the boiler or chiller. These devices have an ideal flow rate (flow rates that are too high or too

low can reduce the system efficiency and possibly damage the equipment). A common pipe connects

the primary loop with the secondary loop to “decouple” the loops, because it hydraulically separates the

primary from secondary loops as seen in the Figure below. In general, the connection is kept as short as

possible to minimize the pressure drop.

Figure 4: Shows the arrangement of the primary and secondary pumps with the heating and cooling systems

See Figure 4 below

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Primary pumps can be a part of primary only or primary/secondary system. In primary only system, the

pumps typically provide a constant design flow through the chiller, set by a balancing valve during

balancing or commissioning. Moreover, three-way valves are often used on cooling or heating coils of

the air handling unit (AHU) to maintain this constant flow through the primary circuit – See Figure 5

below.

Figure 5: Shows the configuration of the three-way valves within the heating and cooling coils

Primary or secondary chilled water systems are generally used in commercial buildings to improve the

efficiency in large central chilled water systems. The primary or secondary pumping system separates

the primary production loop from the secondary distribution loop, which as a result allows the pump in

chillers primary production loop to maintain constant design flow, while the secondary distribution

pumps provide a variable flow as the two-way valves on the chilled water coil varies based on cooling

load demand. Moreover, the primary pump needs to be balances to produce the correct flow. Further to

add, the balancing valve on the

pump discharge is adjusted to

provide correct flow by adding

restriction (pressure drop) to the

circuit

Figure 5: Show the arrangement of

the balancing valve within the

chillers and boilers in the heating

and cooling system

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3.1.5 Secondary Pump

In secondary pumping application the pump draws a variable flow from the primary loop. The flow

through the secondary pump can be less than, equal or greater than the flow in the primary loop. When

the secondary flow is greater than the primary flow some of the returned water in the secondary loop is

pumped back through the decouple pipe without going through the chiller.

3.1.5.1 Traditional Secondary Pumping System

Primary pumps are sized to manage flow requirement and drop in pressure in the production loop only

whereas the larger secondary pumps are sized to circulate the water throughout the system. Decoupled

from the primary pumps,

secondary pumps no longer have

flow constrains and as a result the

system can utilize two way valves

in the cooling or heating coils, and

other energy saving methods

without any problems for the

boiler or chiller.

In a traditional system with two-

way valves as the valves close

when the building loads are

satisfied, the discharge pressure

increases. As the pressure

increases the system resistance

increases and the system curve

rides up the pump curve to a

newer operating point at higher pressure and a lower flow – See Figure 7. Riding the pump curve results

in a slight reduction in pump energy as it

can be seen by comparing the two areas

in Figure 7. As more two-ways valves

close, the discharge pressure continues to

increase. At some point, the system

pressure may be so great that it forces

the two-ways valves to remain open,

even though flow may be further

reduced. As a result, a loss of

temperature control in the building

occurs hence waste of energy.

Figure 6: Shows the two-way valve arrangement with in the cooling and

heating coil

Figure 7: Shows graphically the increase in pressure as the

resistance increases (resistance of flow due to closing of valve)

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3.2 Air Handling Unit (AHU) For Clean Room System The air handling unit (AHU) controls the temperature and the relative humidity in the room. Depending

upon the design requirements, some AHU’s provide 100% fresh outside air while other use recirculation

method.

3.2.1 Filters

Constant air volume is usually supplied and high efficiency filters are added to provide proper room

cleanliness. Two types of efficiency filters may be provided:

1) High-efficiency particulate air (HEPA), which filters with efficiency in excess of 99.97% of 3 micro

particles – See Figure 8a.

2) Ultra low penetration air (ULPA), which filters with a minimum efficiency of 99.999% for 0.12

micro particles – See Figure 8b.

Figure 8: Shows the types of efficiency filter (HEPA & ULPA)

3.2.2 Air Intake Fan

Conventional systems have constant speed fresh air fan, this fan simply pulls the fresh outside air into

the system and through a pre-filter to eliminate much of normal debris and dirt or dust found in the

outside air. In the traditional fresh air intake system the speed remains constant in spite of an increase

or decrease in pressure. As the filter becomes dirty the pressure drop across it increases.

3.2.3 Cooling and Heating Coil

Air handling unit may require providing heating or cooling, or both of them to change the temperature

of air supplied by the intake fan. Temperature is not the only parameter that needs to be controlled in

order to provide a suitable thermal environment. The level of humidity is another important parameter

that affects the people’s definition of suitable thermal environment. Such conditioning (i.e. cooling and

heating) is provided by the installation of heat exchanger coil (s) within the air handling unit’s air stream.

Moreover, these coils may direct or indirect to the medium providing the heating or cooling effect.

Coils are typically manufactured from copper for the tubes, and copper or aluminum for the fin in order

to air heat transfer. In the case of cooling coil, eliminator plates are usually installed to remove and drain

a) b)

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condensate. The hot water or steam is provided by a central boiler, and chilled water is provided by a

central chiller – See Figure 1. Typically, downstream temperature sensors are used to monitor and

control “on/off coil” temperatures, in conjunction with a motorized control valve that is installed prior

to the coil.

If dehumidification is required, then the cooling coil is commanded to over-cool so that the dew point is

reached and condensation occurs. In order to reheat the air to the desired supply temperature, a heater

coil is placed after the cooling coil. This has the effect of reducing the relative humidity level of the

supplied air.

3.3.4 Exhaust Fan

Exhaust fan is used to exhaust air from the clean room to the outside, as this is done for a specific air

change requirements or for a minimum fresh air requirements. Normal flow modulation technique are

used on this fan such as Inlet Guide Vane (IGV) or Discharge Damper Control – See Figure 9a&9b, as the

air is exhausted from the facility and does not have an impact on maintaining the correct class rating of

the clean room.

Figure 9: Shows a) Inlet Guidance Vane flow modulation positions b) Discharge Damper flow modulation positions

a) b)