16 HEAT TRANSFER AND HVAC IN INTERIORS
P.G. Padma Gowri
Introduction:
Heating, ventilation, and air conditioning (HVAC) is applied in the individual buildings or other enclosed spaces the equipment involved in some cases an extension of a larger district heating or district cooling network, or a combined Direct heating and cooling network. In HVAC equipment’s the operating and maintenance aspects are simplified and metering becomes necessary to bill for the electrical energy that is consumed. In some buildings HVAC equipments releases the energy that is reused for other system. For example, at a given time one building may be utilizing cold water for air conditioning and the warm water it returns may be used in another place in building for heating, or for the overall heating-portion of the direct heating and cooling network.
Objectives:
- To understand the concepts of heat transfer in the buildings
- To know various ventilation system used in construction of buildings.
- Understand and apply air condition systems in buildings.
Heat
Heat is one form of the energy. Heat as molecular energy and universally accepted.Since all the forms are converted in to heat it is considered to be energy in its lowest form. Availability of heat energy to do work depends on a temperature differential.
Temperature is a property of matter and it is the measure of intensity of heat or the heat level and it has a relative value. A substance is said to be ‘hot; or cold’ when temperature is compared to some other reference temperature. Heat is not visible like other forms of energy heat can be measured because it has quantity and intensity. In building indoor the ambient temperature is called as room temperature. The outside surrounding air or the weather ambient temperature refers to the current air temperature outside temperature
Heat transfer:
If there is a temperature difference between the conditioned indoor space of a building and outdoor ambient, heat transfer takes place through the building structure like walls, roof, floor etc. This phenomenon is known as heat gain or loss, of the building’s interior .In heat gain in the interior is nothing but heat transfer is from surrounding air to the building .In heat loss in the interior is nothing but heat transfer is from building to the surrounding air.
The heat transfer includes sensible heat transfer through all the structural elements of a building. The commonly known modes of heat flow are conduction, convection and radiation.
Conduction in solids, convection of fluids like liquids ,gas and radiation through anything that will allow radiation to pass through it .
Every material used for construction of buildings has a property of low conduction or high conduction.
- Conduction is heat flow on a molecular scale. Medium at rest or moving;
- Convection is heat conveyed as internal thermal energy of mass that is displaced by mean or turbulent motion.
- Radiation is heat transfer by electromagnetic waves such as infrared or visible light
Requiremants for producing comfort:
Since heating of buildings is normally required to provide comfort for the occupants .Under normal conditions the average human body decipates between 100 to 200 watts per hour, and has a surface temperature of about 27oC. The above quantity of heat about 45 persent is decipated by radiation ,about 30 percrent decipated by convection and the remainder (25% ) by evoperation; the radiaton is coverned by the temperature of the surrounding walls and other objects,the convection by the air temperature,and the evaporation by the humidity of the air.For the commfort of human the temperature of the indoor maintained desired level. however ,surcumstances in which considerable variations from the above can be allowed with out causing discomfort. Maintaing correcty temperature and humidity various technological and decorative aspects are taken in to account.
We design a heating techniques is to aim at maintaining an inside temperature between 17.2 o C and 18.3o C. If it is not desirable,however ,to maintain more than a 8.3 o C to 11oC differeence between indoor and outdoor temperatures. Numerous types of electric heating equipment have been developed ,and those which have found application in practice may be classifed as follows,
1. High temperature radiators
2. Low temperature convertors
3. Pannel heating
4. Air Conditioning
5. Thermal storage.
Heating Systems:
The northern part of India and northern part of countries in the earth has temperature in winter goes below freezing temperature of water. It becomes necessary to heat the houses by supplying the heat from external sources to maintain the comfort condition for the people. Heating systems are required for low temperature places like north India. Proper control of temperature and humidity provides comfort to the human bodies.
To maintain the rooms of the building at the comfortable temperature, it is necessary to supply continuously a definite amount of heat to each room, equal to the amount of heat lost by the room to the surrounding atmosphere.
There are different types of heating systems; all require certain essential elements as given below:
1) A fluid for conveying heat.
2) A compression chamber where a fuel is burned and the heat is given to the fluid used in circulation.
3) Arrangement of pipes for conveying the hot fluid to the hot rooms.
4) The radiators are convectors for dissipating the heat in the rooms.
Classification of Heating System
Warm Air Furnace System
1. Systems providing circulation through pipes
2. Pipe less system
3. Fan and furnace system
Hot Water Heating
1. Natural circulation system
2. Forced circulation system
Stream Heating System
1. High pressure system
2. Vapour system
3. Vacuum system
Warm Air Furnace System
Warm air furnace heating is used natural circulation. The air is heated in the furnace becomes lighter and rises through the ducts to the room. The part of air supply to the room is mixed with the fresh air. The hot air is usually supplied at the floor level. The circulation of the air from floor level to ceiling level is done by proper circulation method.
Advantages of Warm Air Furnace System
1. The capital cost is usually lower than water heating or stream heating system.
2. Simple in operation
3. Low running and maintenance cost
4. Adjustable room temperature
5. Noiseless air circulation.
Disadvantages of Warm Air Furnace System:
1. It cannot used for big buildings, factories and multi storage buildings
2. Equal air distribution through the room is not possible
3. Proper control of humility not possible.
Hot Water Heating:
Hot water is very popular heating medium for central plants. The flow of water in the plant may be produced as a result of density differences between hot and cold water or it can be pumped by putting pump in the system. The expansion tank is placed at highest point of the system. In case the fire becomes hot and causes the water to boil, then the stream will escape through the open top of the expansion tank. The gravity flow circulation arrangement is converted in to forced circulation by addition of pump in to the circuit. The temperature of the water supplied to each radiator is lower in size.
Stream Heating System:
Latent Heat:
Latent heat is the heat energy which is in hidden form and has to be supplied to change the state of a surface without changing its temperature i.e. solid substances melting particular temperature is called latent heat.
Steam heating systems are preferable to the other systems of heating because it carries more quantity of heat per Kg of steam compared with per kg of water or air.
The useful heating is caused mostly by condensation of steam when it is supplied to radiators and convectors.
The quantity of heat given by condensation of steam at atmospheric pressure is equivalent to the value of latent heat of vaporization which is 540 kcal/kg of steam.
The latent heat loss was eliminated by using steam heating system. Pressure value also increased it results less diameters of steam pipe only required. High pressure steam supplied to thedecapitators, more quantity of heat per m2 area of the radiator and requires small diameter pipes.
The radiators capable of heating the building satisfactory in sub-zero temperature would over heat the buildings in mid weather condition.
Classification of Electric Heating Equipment:
High Temperature Radiators
The domestic electric radiator or fire consisting of a resistant element operating at a red heat is called as high temperature radiators. It is an open heat closed resistant elements operating at 66° or 93°C. It is design and located to produce convection currents.
Panel Heating
Large panels of fire clay or other suitable substance in which is incorporated resistance wire. Such panels or mounted on or built into the walls or ceiling and dissipate their heat by radiation.
Air Conditioning
The air circulated through the building by the ventilating equipment is free heated and also if desired treated for control of humidity.
Thermal storage
Hot water is circulated through pipes and radiators, the water being heated electrically. The salient features of this system are that sufficient heat can be stored in the water to allow the current to be switched for several hours.
The Classification of Stream Heating System
1. One pipe gravity return system
2. Two pipe gravity return system
3. Vapour type gravity return system
4. Vacuum system with vacuum pump
Ventilation System
Ventilation:
Ventilation is the movement of air into and out of an interior space by either natural way or mechanical force.
1. Natural ventilation system
The flow of air caused naturally does to the pressure difference between the ambient air and air in an enclosed space is known as natural ventilation system. The flows of air caused do to this known as wind. As wind blows is always uncertain, both in its direction and speed. So there is no control of dust and odour carried by incoming air. The natural ventilation system has no significant in air conditioning plants.
2.Mechanical ventilation system
The control ventilation caused with the use of electrical motors like fan is known as mechanical ventilation system. This is essential when the temperature, humidity and air motion are to be controlled in an air condition space.
3. Extraction system
This is most widely used method in air-conditioning plant. The air from the air conditioned space is withdrawn by the action of suction with the help of suction fan. This is accomplished either by using the propeller fans exhausting through holes in the wall or by means of duct depending upon the requirements. To ensure the uniform fresh air can be combined with extraction system it is called complaining system.
Air Conditioning System:
Air Conditioning system defined as an assembly of different parts of the system used to produce a specified condition of air with in required space of the building.
To select proper air Conditioning system for a given space requirements is difficult task. The designer has to keep in mind the fitness of the system to the space to be conditioned and the classifications of the customer. Selection of air conditioned system economic conditions also plays an important role. Ideal Air Conditioning system should maintain correct temperature, humidity, air purity, air movement and noise level.
Classification of Air Conditioning system:
1) Central station Air Conditioning system
2) Unitary Air Conditioning system.
3) Remote Air-Conditioning Unit System
4) Direct Air Conditioning system
5) Self-contained Air Conditioning units.
Central station Air Conditioning system:
In Central station Air conditioning system, all the components of the system grouped together in one central room and conditioned air is distributed from the central room to the required places through extensive duct work. The Central Air Conditioning system is generally used for the load above 25 tonnes of refrigeration and 2500 m3/min. of conditioned air.
Central station Air Conditioning system require the following components are assembled on the site.
1. Cooling and de humidifying coils.
2. Heating coils
3. Blower with motor
4. Sprays for cooling
5. Air cleaning equipments.,
6. Blower with motor.
Unitary Air Conditioning system:
All the components of Unitary Air Conditioning system assembled in the factory itself. These assembled units are usually installed in or immediately adjacent to a zone or space to be conditioned. It is commonly preferred for 15tons capacity or above or around 200m3/min of air flow. Recently even 100 tones capacity units are also manufactured.
The classification of unitary air conditioned systems:
- Remote Air-Conditioning Unit System
The remote air-conditioning remote is ordinarily separated from the refrigeration condensing unit and heating plant.
- Self-contained Air conditioning unit.
All the components including condensing and heating units are assembled in one enclosure.
- Unitary system for Multi-storey building
This is combination of unitary and central air conditioning system.
District Air –Conditioning System
District cooling system is not as popular as district heating systems. Very recently the use of district cooling system is economically justified. District cooling plants are best suited for concentrated load.
Self contained air conditioning units
Self-contained units are available in wide variety of sizes and for many specific purposes. The following for types are commonly available in the market.
Residential air-conditioning unit.
Air conditioning for homes is becoming increasingly popular n the western countries. Many methods are used, each involving a different type of arrangement of the equipments.
Direct expansion system
In this system a self-contained air-conditioned unit is located in or next to the space to be air conditioned.
Conclusion:
Uniform temperature in an air condition room is the prime requirement of comfort and to identify the efficiency of design of the system. It is important for new building construction designers are to consider heating, ventilation and air conditioning needs with in the building. It is very important to design the Heating, ventilation, and air conditioning (HVAC) system based on the individual building to make sure the equipment is not oversized. There are many different HVAC systems, and each tends to be unique to its building. It is. Adding quality to the envelope and minimizing loads can also reduce the final Heating, ventilation, and air conditioning system cost.
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Web links
- https://en.wikipedia.org/wiki/HVAC mnre.gov.in/solar-energy/ch4.pd
- https://www.worldbuild365.com/
- https://energy.gov/sites/prod/…/sustainable_guide_ch5.pdf