Central Air Conditioning System MCQ Quiz - Objective Question with Answer for Central Air Conditioning System - Download Free PDF

Explanation:

Various reasons and effects of multi-split AC are following.

Reason	Effect
Low voltage	Test lamp or voltmeter indicates the inadequate voltage at the compressor
poor cooling	Dirty air filter
Loss of refrigerant charge	Pressure switch contacts open
Low refrigerant charge.	Compressor cycling on a low-pressure switch
Defective overload.	Compressor cycling on overload
Overcharge of refrigerant.	Higher than normal head pressure

Explanation:

Indirect expansion chiller plant:

• A refrigeration system in which a secondary coolant is cooled by the direct expansion of a primary refrigerant and is then circulated to cool the medium which absorbs heat from the space to be cooled.
• In this system, secondary refrigerant chilled water or brine from the refrigerator is circulated through the cooling coil located in the air handling unit.
• The air handling units comprise the cooling coil through which the chilled brine flow, and the blower.
• The blower sucks hot return air from the room via ducts and blows it over the cooling coil.
• The cool air is then supplied to the space to be cooled through the ducts.
• The brine solution which has absorbed the room heat comes back to the evaporator, gets chilled, and is again pumped back to the air handling unit.
• In this system, sensible heat is absorbed by water while passing through cooling coils and the same heat is taken away by the secondary refrigerant.

Explanation:

Expansion device:

• An expansion valve is a flow regulator that is intended to decrease the pressure of the fluid reaching it by partially vaporizing it, which leads to lowering the temperature of the refrigerant in the case of cooling systems due to an increase in heat transfer surface area.
• Not only does it regulate the fluid temperature, but also it regulates the flow rate to enter the evaporator.

Float valve:

• The float valve is used in a flooded-type evaporator (flooded chillers). 
• In construction, it has a float chamber, float ball, pivoted arm lever, needle, strainer inlet, outlet liquid eliminator, and evaporator coil.
• The operation of the low-side float valve depends upon the rate of evaporation when evaporation increases the liquid level falls below the normal level.
• The float ball moves downwards opens the valve and admits more liquid into the float chamber and evaporator coil.
• Then the liquid level increase and the float ball moves upwards and reduces the flow of liquid refrigerant into the evaporator.
• When the rate of evaporation decreases the liquid level increase in the float chamber and the float valve push the needle and prevent further entry of liquid.

Explanation:

Air ducts in the AC plant:

• Ducts are used to distribute the cool air from the Air Handling Unit or Air Washer, uniformly throughout the building to be air-conditioned.
• They start at the A.H.U. and travel to the spaces to be conditioned carrying the cool air.
• Glass fiber ducting, which is self-insulating is often used in domestic and other small installations, where air velocities do not exceed 10 m/s and duct dimensions are not so large, to require structural supports.
• In commercial and industrial, supply and return ducts are fabricated is galvanized steel or aluminum sheets. 

Galvanized Iron:

• Galvanized iron is essentially iron that’s been coated with a protective zinc layer on the outside.
• Iron itself is susceptible to weather-related degradation.
• When exposed to moisture and oxygen, for example, iron will rust and corrode.
• Over time, the presence of rust and corrosion can eat through the iron, essentially jeopardizing its structural integrity.
• The zinc coating resists rust and corrosion in galvanized iron.

Explanation:

Cooling Towers:

• Cooling towers are an integral component of many refrigeration systems, providing comfort or process cooling across a broad range of applications.
• Cooling towers are used to reject heat through the natural process of evaporation.
• Warm recirculating water is sent to the cooling tower where a portion of the water is evaporated into the air passing through the tower.
• As the water evaporates, the air absorbs heat, which lowers the temperature of the remaining water.
• Cooling towers are classified by the direction of airflow (counter-flow or cross-flow) and the type of draft.
• Natural
• Mechanical 
  1. Forced draft cooling tower
  2. Induced draft cooling tower

Induced draft cooling tower

• This type of cooling tower comes under the mechanical draft cooling tower.
• In construction, it has a water tank, float valve, water circulating pump, spray nozzles, eliminator, and fan.
• The bottom has a water tank and the water level is maintained by a float valve.
• The makeup water inlet is connected to the float valve.
• The water out let is connected to the water circulating pump.
• The pump outlet is connected to the spray nozzles. through condenser. 
• The fan is located on the top of the cooling tower.
• The eliminators are fitted just below the fan and spray nozzles are fitted just below the eliminator.
• Induced draft towers tend to be larger than forced draft units and are used for large-capacity chiller plants.

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Additional Information

​Natural Draft towers:

• Natural draft towers are designed to move air up through the structure naturally without the use of fans.
• They use the natural law of differing densities between the ambient air and warm air in the tower.
• The warm air will rise within the chimney structure because of its lower density drawing cool ambient air in the bottom portion.
• Often times these towers are very tall to induce adequate airflow and have a unique shape giving them the name “hyperbolic” towers.

Recovery of refrigerants: Refrigerants like CFCs & HCFGs cannot be vented into the atmosphere as they cause ozone depletion in the stratosphere and also contribute to global warming. So, removing these refrigerants in any condition from an appliance and store in an external container is called recovery.

Methods of recovery: There are two methods

1. Passive Methods (No external recovery machine used): In this movement of refrigerant takes place due to the natural difference between system and recovery cylinder. This process is speeded up by following methods

• Evacuating the recovery cylinder
• Place recovery cylinder in an ice bath
• Supply heat to the system

2. Active methods of recovery

Special recovery cylinders: Special recovery cylinder with float switch which will stop the recovery until when the liquid level exceeds 80% of the volume.

Central AC plants have capacity higher than 30 tons. These plants are employed in industries, hotels, big buildings etc. On the other hand, the capacity of window and split air conditioners are usually up to 5 tons.

Recovery of refrigerants: Refrigerants like CFCs & HCFGs cannot be vented into the atmosphere as they cause ozone depletion in the stratosphere and also contribute in global warming. So, removing these refrigerants in any condition from an appliance and store in an external container is called recovery.

Methods of recovery: There are two methods

1. Passive Methods (No external recovery machine used): In this movement of refrigerant takes place due to natural difference between system and recovery cylinder. This process is speeded up by following methods

• Evacuating the recovery cylinder
• Place recovery cylinder in ice bath
• Supply heat to the system

In the given figure, the cylinder is places in ice bath which will speed up the recovery.

2. Active methods of recovery

HVAC stands for “heating, ventilation and air conditioning”. The HVAC systems have become the required industry standard for construction of new buildings.

Types of HVAC systems: Three types of HVAC systems are available today.

1. Split and window AC

2. Packaged Heating and Air Conditioning System

3. Central AC system

Direct Expansion (DX) system: In this system, the air from the space to be air conditioned is circulated over the cooling coil.

Indirect Expansion system: In this system, secondary refrigerant chilled water or brine from the refrigerator is circulated through the cooling coil located in the air handling unit.

Air Handling Unit (AHU) is used in the central AC plants. This unit consists of a blower, heating and cooling elements, filter racks or chamber, dampers, humidifiers and other central equipment in direct contact with the air flow. This does not include the ductwork through the building.

Function of duct is to convey the air between two points. It supplies air from handling unit to the room to be air conditioned. Pressure drop takes place in the duct due to the friction losses and sudden direction changes. That’s why galvanised iron or aluminium sheets are preferred for duct because they have smooth surface and thus offer less resistance to air flow. Usually rectangular ducts are preferred with aspect ratio of 4 : 1.

The Air Handling Unit (AHU) is a centrifugal type of blower or the fan usually located in the chilled water coil cabinet. Its purpose is to create a pressure differential, so that the air from the conditioned space is drawn to the unit.

The drain line is on the suction side of the fan. As the surrounding air is sucked through drain pipe it contains outside foul smell. This foul smell is avoided by U Trap in the condensate line. The height of U trap should be more than the negative pressure created by the fan in the fan chamber.

The Fan Coil Unit (FCU) is used for independent cooling of the rooms via central air conditioning system. FCU is a sheet metal cabinet. This houses a chilled water coil, a blower with motor and an air filter.

The water is chilled centrally and pumped to various parts of the building through insulated pipes. The chilled water enters to FCU, where heat exchange takes place between the room air and the chilled water in the coil.

In central air conditioning plant, all the major components like compressor, condenser, chiller, water pumps and controlling devices are placed at one place. The cool air is circulated to all places via ducts. The fan and blowers help in circulating this air.