NOVEMBER / DECEMBER 2012

PART-A

1. Describe the significance of thermodynamics and its application.

Thermodynamics is a branch of physics concerned with heat and temperature and their relation to energy and work including pressure, density etc. in a substance. Thermodynamics focuses on how heat transfer is related to various energy changes within a physical system. The laws of thermodynamics dictate the specifics for the movement of heat and work.

Zeroth law of Thermodynamics. :

It's about thermal equilibrium. When two objects or systems are in thermal equilibrium with each other, the heat energy flowing from the first object to the second is the same as that flowing from the second object to the first. Hence they are at the same temperature.

First Law of Thermodynamics states : Energy can't be created or destroyed, but can be converted from one form to another. The total amount of energy and matter in the Universe remains constant

Second law of thermodynamics states : Not all energy can be used. It states that as the disorder in the universe increases, the energy is transformed into less usable forms. Thus, the efficiency of any process will always be less than 100%. Cooling systems are constructed to operate base on the second law of Thermodynamics.

Third law of thermodynamics states : At absolute zero there is NO energy, and therefore the atoms in a crystal do not move at all. If they are not moving, there is no tendency towards disorder. At all other temperatures, atoms are still vibrating.

2. Explain about the function of Air handling units. Already dealt earlier.

3. State different air conditioning systems for small buildings.

Broadly the airconditioning system for small building can be grouped as Unitary or Semi-Central.

Unitary System : These basically light duty systems meant to cool limited spaces not exceeding 400 sq.ft of a single zone. They can be either Window Airconditioners or Split Airconditioners. Further in split AC we have many versions like High Wall, Cassette, Vertical, Concealed etc to suit the application. Both have limited air distribution facility and hence using multiple units for larger areas is not the correct choice.

Semi – Central : For fairly large areas to cool more than 400-500 sq.ft we can consider Semi-Central systems. They can be either floor mounted packaged air-conditioners or ceiling suspended ductable split ACs. They units produce cold air and unlike the unitary ( which throws the cold air ), these units actually distribute the air through ducting and grilles / diffusers.

WINDOW AC

SPLIT AC

PACKAGED UNIT

DUCTABLE SPLIT UNIT

4. Explain the function of chilled water plant. Already dealt earlier.

5. Describe the design issues to be addressed in horizontal distribution systems.

The horizontal distribution system for mechanical and electrical services in a large building should be planned simultaneously with the structural frame and the interior finish systems, because the three are strongly interrelated. The floor–to-floor height of a building is determined in part by the vertical dimension needed at each story for horizontal runs of ductwork and piping. The selection of finish ceiling, partition, and floor systems is often based in part on their ability to contain the necessary electrical and mechanical services and to adjust to future changes in these services. All these strategies involve close cooperation among the architect and the structural and mechanical engineers.

6. State the choices of cooling systems for large buildings. Already dealt earlier.

PART-B

7. (a). Describe in detail about Basic refrigeration principles and its application.

Principles of Refrigeration :

Liquids absorb heat when changed from liquid to gas Gases give off heat when changed from gas to liquid.

For an air conditioning system to operate with economy, the refrigerant must be used repeatedly. For this reason, all air conditioners use the same cycle of compression, condensation, expansion, and evaporation in a closed circuit. The same refrigerant is used to move the heat from one area, to cool this area, and to expel this heat in another area.

The refrigerant comes into the compressor as a low-pressure low-temp gas, it is compressed and then moves out of the compressor as a high-pressure high -temp gas.

The gas then flows to the condenser. Here the gas condenses to a liquid, and gives off its heat to the outside air. There is only change of phase and no change in pressure or temp.

The liquid then moves to the expansion valve under high pressure high-temp. This valve restricts the flow of the fluid, and lowers its pressure and temp as it leaves the expansion valve.

The low-pressure low temp liquid then moves to the evaporator, where heat from the inside air is absorbed and changes it from a liquid to a gas. There is only change of phase and no change in pressure or temp.

The refrigerant moves to the compressor where the entire cycle is repeated.

Note that the four-part cycle is divided at the center into a high side and a low side. This refers to the pressures of the refrigerant in each side of the system

(Or)

(b). Elaborate in detail the significance of refrigerants control devices and functions.

THERMOSTATIC EXPANSION VALVES - TXV is the most widely used refrigerant flow control device. This valve controls the refrigerant flow into the evaporator. It is called thermostatic because they are responsive to the temperature of the refrigerant which again depends on the demand for cooling.

When there is an increase in the demand for cooling in the evaporator the pressure increases. This is sensed by the TXV which automatically opens the the valve to let more refrigerant into the evaporator. The reverse happens when there is a drop in demand.

Once the refrigerant from the compressor gets enters the expansion valve ,the high pressure and high temp liquid runs into a constriction that doesn't allow the refrigerant to pass through easily. As a result, when the liquid does get through to the other side which is expanded, it finds itself in a much lower pressure. When the pressure drops like this, so does the temperature. Now this cold liquid cools the air and thus we get air-conditioned air.

THERMOSTATIC EXPANSION VALVE

8. (a). Explain the design criteria for selecting the air conditioning systems for buildings with energy conservation measures.

The following steps have to be undertaken :

Proper heat load calculations will have to be prepared so that correct capacity of the system can be arrived at. Whenever we select an air-conditioner based on the heat load calculations we have to ensure that the capacity as well as the air quantity required as per the heatload is matching with the equipment being selected.

In the heatload itself we can find ways and means to reduce the heat ingress like selecting heat resistant glass, insulation for walls and roof, fresh air quantity as per ASHRAE standards, LED hights, fenestrations etc.

Based on the study of the application involved we have to select the correct type of system i.e whether Unitary, Semi-Central or Central.

After the system is selected we have to incorporate materials, systems and controls to ensure that less energy is utilized. Some of them are :

Unitary systems :

Star Rating : Out of 5 / 3 / 2 star select the 5 star. Between 5 and 2 star there is 30 % saving. Still better is to go in for Inverter based air-conditioner which assures additional 25 % power saving. This is because the inverter changes the speed of the compressor and ensures that only the exact cooling required is produced by the compressor.

Semi-Central :

Consider Energy efficient scroll compressors. Also go in for multiple compressors wherein the compressor is cut-off in case of part load operations.

Go for Energy efficient motors. Go for high performance units wherein the indoor and outdoor unit copper

coil is sized 40 % more. This additional area ensures that more heat is rejected ( condenser ) and more heat is absorbed ( evaporator ) leading to less strain on the compressor and saving 20 % energy.

Central :

Consider going in for Screw chillers which are more energy efficient than others.

While comparing the chillers , select the chiller based on its efficiency at part load conditions and not in the full load conditions because full load happens only 1 % in a year.

Ensure that the ancillary equipments like pumps, cooling tower, blowers etc also respond to the part load conditions. Use Variable Frequency drives for this purpose.

Materials and Equipments :

Whenever a motor is involved go in for Variable Frequency Drives – VFD. Go for microprocessor controls instead of manual controls which ensure

immediate response to part load conditions. Go for Building Management System – BMS.

For air-distribution go for factory made ducting than site fabricated ducting. Ensure design is based on low pressure drop in piping and ducting system. Consider latest insulation materials like elastomeric in place of fiberglass /

expanded polystyrene. Go for pre-insulated pipes in place of site insulation. Go for double skin AHU in place of single skin AHU. Go for direct drive plug fans in place of belt driven centrifugal fans. Go for VAV – Variable Air Volume systems to control air in smaller rooms

which are part of a bigger office. Go for Energy Recovery Ventilator – ERV for maintaining IAQ as per

standard and still recovering 70 % energy.

(Or)

(b). Explain the requirement for chiller plant and its supporting components of design.

CHILLER PLANT

In a chilled water system the chiller plant chills water. Then the water is pumped to the cooling coil in the Air Handling Unit – AHU. The chilled water then cools the air. This is an indirect system of cooling.

Water can be easily pumped for long distances without any loss. In case of multistoried buildings or multiple buildings a chilled water system with a single air-conditioning plant, it is possible to air-condition the entire building with multiple air handling units and fan coil units. This also gives flexibility of usage.

Capacities available are from 10 to 1500 TR in single machine, multiple units can be used for larger installations. And available in air cooled and water cooled versions with Scroll, Screw and Centrifugal type Compressors.

Advantages are these types of machines can handle easily system diversities thus reducing the total installed capacities like in hotels, offices and multi storied

buildings. They have a better response to part loads offering better control, very rugged systems, better humidity control as compared to packaged and split units.

Disadvantages are slightly longer lead times, extensive site work,costlier than DX systems, packaged / split units especially for small jobs. Also needs trained manpower to operate.

While designing a total system based on chiller, care has to be taken to ensure that related equipments like pumping system, piping system, air handling units, air distribution system, control systems etc are properly designed.This ensures that the entire system consumes less energy.

9. (a). Elaborate in detail the process of horizontal distribution of services large buildings.

The horizontal distribution system for mechanical and electrical services in a large building should be planned simultaneously with the structural frame and the interior finish systems, because the three are strongly interrelated. The floor–to-floor height of a building is determined in part by the vertical dimension needed at each story for horizontal runs of ductwork and piping. The selection of finish ceiling, partition, and floor systems is often based in part on their ability to contain the necessary electrical and mechanical services and to adjust to future changes in these services. All these strategies involve close cooperation among the architect and the structural and mechanical engineers.

Sometimes the major runs of ductwork, piping and wiring can be grouped in the ceiling area above the central corridor of each floor of a building, leaving the ceilings of the surrounding rooms essentially “clean”. This works especially well in hotels, dormitories, and apartment buildings that rely on above ceiling all-water system. A low corridor ceiling is readily accepted in exchange for high, unobstructed space in the occupied rooms, where the structure may be left exposed as the finish ceiling, saving cost and floor-to-floor height.

In broad expanses of floor space, particularly where all electrical and communications services must be available at any point in the area, an entire horizontal layer of space is reserved on each story for mechanical and electrical equipment.

Above a suspended ceiling, all services can be taken like :

Fire Sprinkers Electrical wiring Communication and

Data

Plumbing HVAC piping HVAC ducts ( supply air, return air,

fresh air and exhaust air )

(Or)

(b). Describe the process of selection and adopting the type of choice cooling systems.

UNITARY SYSTEM :

When to use : If room size does not exceed 400 sq.ft. If used only for light work like residences , small office rooms etc.

When not to use : If requirement is heavy duty Don’t use multiple units for areas above 500 sq.ft. When standards need to be followed

SEMI CENTRAL :

When to use : If requirement does not exceed 4000 sq.ft. If used only for light commercial applications like offices, showrooms etc. Single zone areas.

Fresh air can be handled.

When not to use : If requirement is high sensible or high latent load. Multizone areas.

CENTRAL PLANT – DX :

When to use : If requirement exceeds 4000 sq.ft. For heavy duty offices, showrooms, industries etc. Single zone areas.

When not to use : Multizone areas. When copper pipe length is more than 50 feet.

CHILLER :

When to use :

If requirement does exceeds 2000 sq.m.

If used for heavy duty commercial and industrial applications like IT Parks, Star Hotels, Hospitals, Major offices, Major showrooms, Industries, non-standard application etc.

Multizone Zone areas.

If plant has to be located far way from usage

When not to use :

If requirement is highly variable and may go down to less than 40 %.