HVAC acronym stands for “Heating, Ventilating and Air Conditioning”. Three functions often combined into one system in today's modern homes and buildings. Warmed or cooled or dehumidified air flows through a series of tubes - called ducts - to be distributed to all the rooms of your house.

            HVAC is sometimes referred to as climate control and is particularly important in the design of medium to large industrial and office buildings such as skyscrapers and in marine environments such as aquariums, where humidity and temperature must all be closely regulated whilst maintaining safe and healthy conditions within.

            Heating, ventilating, and air conditioning is based on the basic principles of thermodynamics, fluid mechanics, and heat transfer, and two inventions and discoveries made by Michael Faraday, Willis Carrier and many others. The invention of the components of HVAC systems goes on modernization and higher efficiency, and system control are constantly introduced by companies and inventors all over the world.

            HVAC systems have improved in energy efficiency in the last decade. As a result, you can save money and increase your comfort by properly maintaining and upgrading your HVAC equipment.

            The three functions of heating, ventilating, and air-conditioning are closely interrelated. All seek to provide thermal comfort, acceptable indoor air quality, and reasonable installation, operation, and maintenance costs. HVAC systems can provide ventilation, reduce air infiltration, and maintain pressure relationships between spaces. How air is delivered to, and removed from spaces is known as room air distribution. the all three functions which are used in HVAC are elaborated.




1) Heating

            Heating systems may be classified as central or local. Central heating is often used in cold climates to heat private houses and public buildings. Such a system contains a boiler, furnace to heat water, steam, or air, all in a central location such as a furnace room in a home or a mechanical room in a large building. The system also contains either ductwork, for forced air systems, or piping to distribute a heated fluid and radiators  to transfer this heat to the air.

            In boiler or radiant heating systems, all but the simplest systems have a pump to circulate the water and ensure an equal supply of heat to all the radiators. The heated water can also be fed through another (secondary) heat exchanger inside a storage cylinder to provide hot running water.

            Forced air systems send heated air through ductwork. During warm weather the same ductwork can be used for air conditioning. The forced air can also be filtered or put through air cleaners.

            Heating can also be provided from electric, or resistance heating using a filament that becomes hot when electricity is caused to pass through it. This type of heat can be found as backup or supplemental heating for heat pump system.

            The heating elements should be located in the coldest part of the room, typically next to the windows to minimize condensation and offset the convective air current formed in the room due to the air next to the window becoming negatively buoyant due to the cold glass. Devices that direct vents away from windows to prevent "wasted" heat defeat this design purpose.



2) Ventilating

            Ventilating is the process of "changing" or replacing air in any space to control temperature or remove moisture, odors, smoke, heat, dust and airborne bacteria. Ventilation includes both the exchange of air to the outside as well as circulation of air within the building. It is one of the most important factors for maintaining acceptable indoor air quality in buildings.

            Methods for ventilating a building may be divided into mechanical/forced and natural types. Ventilation is used to remove unpleasant smells and excessive moisture, introduce outside air, and to keep interior building air circulating, to prevent stagnation of the interior air.

Mechanical or forced ventilation

           "Mechanical" or "forced" ventilation is used to control indoor air quality. Excess humidity, odors, and contaminants can often be controlled via dilution or replacement with outside air. However, in humid climates much energy is required to remove excess moisture from ventilation air.

            Ceiling fans and table/floor fans circulate air within a room for the purpose of reducing the perceived temperature because of evaporation of perspiration on the skin of the occupants. Because hot air rises, ceiling fans may be used to keep a room warmer in the winter by circulating the warm stratified air from the ceiling to the floor. Ceiling fans do not provide ventilation as defined as the introduction of outside air.

Natural ventilation

            Natural ventilation is the ventilation of a building with outside air without the use of a fan or other mechanical system. It can be achieved with operable windows when the spaces to ventilate are small and the architecture permits. In more complex systems warm air in the building can be allowed to rise and flow out upper openings to the outside thus forcing cool outside air to be drawn into the building naturally through openings in the lower areas. These systems use very little energy but care must be taken to ensure the occupants' comfort. In warm or humid months, in many climates, maintaining thermal comfort via solely natural ventilation may not be possible so conventional air conditioning systems are used as backups. Air-side economizers perform the same function as natural ventilation, but use mechanical systems' fans, ducts, dampers, and control systems to introduce and distribute cool outdoor air when appropriate.

            Another development of the 1990s is the whole house approach to heating and cooling. Coupled with an energy efficient furnace, heat pump or air-conditioner, the whole house approach can have a great impact on your energy bills. By combining proper equipment maintenance and upgrades with appropriate insulation, weatherization and thermostat settings - properly regulated with a programmable thermostat, of course - you may be able to cut your energy bills in half.

 Various Types of HVAC Systems

      1)   All- Air System

2)      Central Air Conditioning system

3)      Cogeneration System and Heat Recovery System

4)      Hydronics System



1) All- Air Systems


            All-Air System provides complete sensible and latent preheating and humidifying capacity in the air supplied by the system. No additional cooling or humidification is required at the zone, except in the case of certain industrial systems. Heating may be accomplished by air stream; in some application heating is accomplished by separate heater.

All –Air Systems classified in two categories.

I) Single Duct Systems.

II) Dual Duct Systems.




I)Single Duct Systems.

            Single Duct System contains main heating and cooling coils in a series flow air paths.

Single Duct System is further divided as fallow


    i) Constant Volume

it includes 1)Single zone

                  2) Multiple zone reheater

                  3) Bypass

    ii) Variable Air Volume (VAV) System

II) Dual Duct Systems

            Dual Duct Systems contains main heating and cooling coils in parallel flow or series-parallel flow air paths with either

1) A separate cold and warm air duct distribution system that blends the air at the terminal apparatus (Dual Duct System).

2)A separate Supply Air Duct to each zone with the supply air blended to the required temperature at the main unit mixing dampers.(Multizone).




Three basic methods are available for cooling

1) Direct expansion which takes advantage of latent heat of fluid.

2) Fluid filled coil where temperature difference between the fluid and the air cause an exchange of energy

3) Direct spray of water in the air stream in which an adiabatic process uses the latent heat of evaporation of water by spraying chilled water, sensible and latent cooling also possible. In evaporative cooler recirculated water is sprayed on drips onto a filter pad




1) Steam, which uses the latent heat of fluid.

2) Fluid filled coils, which uses temperature differences bet the warm fluid and the cooler air

3)A electric heating ,which also uses temperature difference between the heating coil  and the air to exchange energy.



Fallowing methods are available for the humidification of air

1) Direct Spray of recirculated water into the air stream (air washer) which reduces the dry bulb temperature while maintaining an almost constant wet bulb in an adiabatic process, the air may also be cooled, dehumidified, heated and humidified by changing the temperature of the spray water.

            In one variation, the surface area of the water exposed to the air is increased by spraying water on to a cooling/heating coil. The coil surface temperature determines the leaving air condition.

           Another method is to spray or distribute water over a porous medium. Such as those in evaporative coolers and commercial green houses. This method requires careful monitoring of the water condition to keep biological contaminants from the air stream

2) The use of compressed air to force water through a nozzle in to the air stream which is essentially a constant wet bulb (adiabatic process) The water must be treated to keep particulates from entering the air stream and contaminating or coating equipments and furnishing. Many types of nozzles are available each with different characteristics.

3)Steam Injection which is a constant dry bulb process however as the steam injected becomes super heated, the leaving dry bulb temperature increases when live steam is injected into the air stream, the boiler water treatment chemical used must be non toxic to the occupants and, if the air supplying a laboratory to the research under way.




Moisture will condense on cooling coil when its surface temperature is below the due point of the air, thus reducing the humidity of the air, air will also be dehumidified if fluid with a temperature below the air stream dew point is spread into the air stream. Chemical dehumidification involve either passing air over a solid desiccant or spraying the air with a solution of the desiccant and water, both processes add heat. The air being dehumidified to either precondition (cooled, humidified etc) primary air or recirculated room air



1) The location of the central mechanical room for major equipment allows better operation and maintenance, and also vibration and noise control

2) These system offers the greatest potentials for use of out side air instead of mechanical refrigeration for cooling

3) Seasonal change over is simple and adopts readily to automatic control

4) A wide choice of flexibility and humidity control under all operating condition is possible with the availability of simultaneous heating and cooling even during off season periods

5) Air to Air and other heat recovery may be readily incorporated.

6) The systems are well suited for application of negative or positive pressurization.

7) All Air Systems adapts well to winter humidification.



1) They require additional duct clearance, which reduce usable floor space.

2) Depending on layout larger floor plans are necessary to allow enough space for vertical shafts required for air distribution.

3) Air balancing particularly on large systems, can be more difficult.

4) Perimeter heating is not always available to provide temporary heat during construction.

2)  Central Air Conditioning systems


           Central air conditioning units are usually matched with a gas or oil furnace to provide heat through the same set of ducts. A central HVAC system is the most quiet and convenient way to cool an entire home.

            There are also central HVAC units called heat pumps that combine both the heating and cooling functions. If you heat your home with electricity, a heat pump system is the most efficient unit to use in moderate climates. It can provide up to three times more heating than the equivalent amount of electrical energy it consumes. A heat pump can trim the amount of electricity you use for heating as much as 30 percent to 40 percent.

            Even though air conditioners and heat pumps require the use of some different components, they both operate on the same basic principles. Heat pumps and most central air conditioners are called "split systems" because there is an outdoor unit (called a condenser) and an indoor unit (an evaporator coil). The job of the heat pump or air conditioner is to transport heat from one of these units to the other. In the summer, for example, the system extracts heat from indoor air and transfers it outside, leaving cooled indoor air to be recirculated through your ducts by a fan. A substance called a refrigerant carries the heat from one area to another.


            The compressor in your outdoor unit will change the gaseous refrigerant into a high temperature, high-pressure gas. As that gas flows through the outdoor coil, it loses heat. That makes the refrigerant condense into a high temperature, high pressure liquid that flows through copper tubing into the evaporator coil located in your fan coil unit or attached to your furnace.

            At that point, the liquid refrigerant is allowed to expand, turning the liquid refrigerant into a low temperature, low pressure gas. The gas then absorbs heat from the air circulating in your home's ductwork, leaving it full of cooler air to be distributed throughout the house. Meanwhile, the low temperature, low pressure refrigerant gas returns to the compressor to begin the cycle all over again.

            While your air conditioner or heat pump cools the air, it also dehumidifies it. That's because warm air passing over the indoor evaporator coil cannot hold as much moisture as it carried at a higher temperature, before it was cooled. The extra moisture condenses on the outside of the coils and is carried away through a drain. The process is similar to what happens on a hot, humid day, when condensed moisture beads up on the outside of a glass of cold lemonade.

            The same process works in reverse in a heat pump during the winter. The heat pump takes heat out of the outside air or out of the ground, if you have a geothermal heat pump and it moves that heat inside, where it is transferred from the evaporator coil to the air circulating through your home.

            That's not a typographical error, by the way- the heat pump moves heat from outside to warm your home, even on a cold day. That's because "cold" is a relative term. Air as cold as 30 degrees still contains a great deal of heat - the temperature at which air no longer carries any heat is well below -200 degrees Fahrenheit. A heat pump's heat exchanger can squeeze heat out of cold air, then transfer that heat into your home with the help of a fan which circulates the warm air through your ducts.

            Heat pumps are often installed with back-up electric resistance heat or a furnace to handle heating requirements when more heat is needed than the heat pump can efficiently extract from the air.



Types of Systems

            A "split system" - the condensing unit is placed outside the house, and the evaporator coil is inside. There is another configuration called a "packaged" air conditioner that combines the condensing unit and the evaporator coil into one outdoor unit. Which type you should choose depends on your home's location and construction.

Rating a Unit's Efficiency

            The efficiency of central air conditioning systems is rated by a Seasonal Energy Efficiency Ratio (SEER). SEER ratings typically range from 13 to 23, with the highest numbers indicating the most efficient units that offer the most energy savings year after year. Fortunately, great strides have been made in the last 10 years to increase the efficiency of new air conditioners and heat pumps.

            To be considered as high-efficiency units, air conditioners must have a SEER rating of at least 14. The SEER rating is usually shown on a yellow and black Energy Guide sticker attached to the outside unit of the air conditioner.

        Central air conditioners that are in the top 25 percent of efficient models may carry the Energy Star® label. To qualify, they must have a minimum SEER efficiency level of 14. Additionally, Energy Star® models must also have a minimum Energy Efficiency Ratio (EER) of at least 11.5 for split systems, and of at least 11.0 for single-package models. Air conditioners that bear the Energy Star® label may be twice as efficient as some existing systems. Heat pumps also have heating efficiency ratings, indicated as a Heating Seasonal Performance Factor (HSPF). In general, the higher the HSPF rating, the less electricity the unit will use to do its job. High-efficiency central air-conditioning heat pumps can also qualify as Energy Star? models. In addition to meeting the minimum SEER and EER requirements, they must also meet minimum HSPF requirements of 8.2 for split systems and 8.0 for single-package models.

            Higher efficiency units usually cost more to purchase initially, but save money in the long run on operating costs.

Sound Levels

            Few people think about how loud an air conditioner or heat pump will be - at least until the unit is installed and running in their back yard. With some units, the noise created by the condensing unit outside can even interfere with indoor peace and quiet.        That's why you should compare the sound levels produced by different models when you are shopping for a new unit.

            The sound level of outdoor units is measured in bels (a term similar to decibels). The rating scale goes from 0 - the rating for a barely perceptible sound - to 13 - the threshold of pain. Most air conditioners and heat pumps operate in the range of 8 to 9 bels, although some are quiet enough to rate as 6.8 bels. (While that may not sound like a wide range, consider this: the noise output at 9 bels is 10 times louder than 8 bels. That means one 9-bel air conditioner is as loud as 10 units rated at 8 bels!)

3) Cogeneration System and Heat Recovery System


            The growing worldwide demand for less polluting forms of energy has led to a renewed interest in the use of cogeneration technologies in the residential sector due to their potential for significantly reducing the quantities of pollutants emitted in supplying residential electricity and heating. Cogeneration systems in the residential sector have the ability to produce both useful thermal energy and electricity from a single source of fuel such as oil or natural gas. This means that the efficiency of energy conversion to useful heat and power is potentially significantly greater than by using the traditional alternatives of boilers or furnaces and conventional fossil fuel fired central electricity generation systems. If managed properly this increased efficiency can result in lower costs and a reduction in greenhouse gas emissions. Cogeneration also has the added advantage of diversifying electrical energy production, thus potentially improving security of energy supply in the event of problems occurring with the main electricity grid.The growing worldwide demand for less polluting forms of energy has led to a renewed interest in the use of cogeneration technologies in the residential sector due to their potential for significantly reducing the quantities of pollutants emitted in supplying residential electricity and heating.

            According to the definition given by the American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE), Cogeneration is the simultaneous production of electrical or mechanical energy (power) and useful thermal energy from a single energy stream such as oil, coal or natural gas. In some cases, the energy source can be provided from solar, geothermal, biomass or other type of renewable energy source. There is growing potential for the use of cogeneration systems in the residential sector because they have the ability to produce both useful thermal energy and electricity from a single source of fuel such as oil or natural gas. In cogeneration systems, the overall efficiency of energy conversion can increase to over 80% based on Higher Heating Value for the fuel if all the heat produced can be usefully used, compared to an average of 30 – 35% at the point-of-use for electricity produced in conventional fossil fuel fired electricity generation systems and to an average of 80 - 95% for heat produced in boilers. Cogeneration might not always be the most energy efficient and/or environmentally friendly solution, e.g. where highly efficient centralized electricity generation systems are available; where renewable energy generated electricity is available; etc., so it is important that local energy supply options are carefully appraised when considering installing cogeneration on energy efficiency or environmental grounds.

            The concept of cogeneration can be related to power plants of various sizes ranging from small scale for residential buildings to large scale cogeneration systems for industrial purposes to fully grid connected utility generating stations. End-users that can benefit most from cogeneration are those that can fully use both the electricity and heat energy produced by the system. Consequently, cogeneration is suitable for building applications provided that there is a demand for the heat energy produced.

Building applications suitable for cogeneration include hospitals, leisure facilities (particularly those incorporating swimming pools), institutional buildings, hotels, office buildings and single- and multifamily residential buildings. In the case of single-family applications, the design of systems poses a significant technical challenge due to the potential non-coincidence of thermal and electrical loads, necessitating the need for electrical/thermal storage or connection in parallel to the electrical grid.

            However, cogeneration systems for multi-family, commercial or institutional applications benefit from the thermal/electrical load diversity in the multiple loads served, reducing the need for storage.

Cogeneration applications in buildings can be designed to:

Satisfy both the electrical and thermal demands,

Satisfy the thermal demand and part of the electrical demand,

Or satisfy the electrical demand and part of the thermal demand

Or, most commonly satisfy part of the electrical demand and part of the thermal demand.

            In addition, cogeneration in buildings can be designed for peak shaving applications, i.e. the Cogeneration plant is used to reduce either the peak electrical demand or thermal demand. With each of these potential system designs there are constraints on the practical and economic Viability:

1) For a cogeneration unit designed to fully meet the electrical demand of the building; if the heat demand is less than the thermal output from the cogeneration plant, the plant unit will either throttle back to operate under part load conditions, or will switch on and off, or will require that the surplus heat is dumped to atmosphere/stored in a thermal storage device such as the heat distribution system, the building structure or in water or phase change materials. On the other hand, if the heat demand of the building is higher than the cogeneration capacity, a secondary heat raising system such a boiler is often used to ‘top-up’ the heat output.

2) For a cogeneration unit designed to fully meet the thermal demand of the building; if the electrical demand of the building is less than the electrical output from the cogeneration plant the cogeneration unit can either be throttled back, or the surplus electricity produced can be exported to the utility grid or possibly stored in an electrical storage device such as batteries or capacitors. On the other hand, if the electrical demand of the building is higher than the output of the cogeneration plant, the lack of electricity is usually covered by importing electricity from the utility grid.

3) The economic viability of such systems is critically dependent on the installed cost of each system, system maintenance costs and retail prices for the cogeneration system fuel and centrally generated electricity as well as the electricity exportation price if electricity is exported to the grid. The economic viability of cogeneration in the residential sector benefits from the much higher retail prices paid by residential consumers for grid supplied electricity, though currently this is usually more than offset by the high cost of the cogeneration systems per kWe and kWth. Cogeneration systems are financially more attractive in periods of high electricity prices and low fossil fuel prices. Due to its higher specific investment cost, a careful cogeneration system design procedure is needed in order to define the best sizes of the equipment in the system, accounting for not only the cogeneration equipment but also the heat storage devices and the advanced control systems that will forecast the heat requirement and decide the optimal control using model based predictive control algorithms.

4) To meet the full electrical or thermal demand of a building using cogeneration it is usually necessary to install cogeneration systems which are oversized in both their electrical and thermal outputs. Unless there is a use outside the building for the surplus heat and power this usually has the unwanted consequence that the unit’s running time will decrease due to an insufficient load being available. This reduction in run hours will make the economics of the system poorer. For this reason, cogeneration devices are usually sized to meet only a part of the electrical and thermal need.

Currently, in both residential and commercial sectors, buildings are being built with high levels of insulation, which helps in reducing the space-heating requirements. Heat demand in buildings often follows both daily and seasonal variations due to behavioral pattern of the inhabitants. Presently, several manufacturers have developed products or are developing products suitable for residential or small-scale commercial cogeneration applications like hospitals, leisure facilities, (particularly those incorporating swimming pools), hotels or institutional buildings.






            Cogeneration systems are required to have high annual usage, usually with extensive periods of almost continuous operation in order to be profitable. Factors such as unscheduled outages that lead to high maintenance costs, the inconvenience caused by switching supply source and arranging or getting service engineer to investigate and correct faults, and costs associated with buying energy at unfavorable tariffs reduces the performance of cogeneration systems5. Thus, the performance of a cogeneration system is commonly measured in terms of its efficiency, reliability, availability, maintenance requirements and emissions.


Cogeneration Technologies For Residential


            Cogeneration, or combined heat and power (CHP) technology, is the combined production of electrical power and useful heat. In electricity generation from fossil fuels, the waste heat can be recovered from the cooling water and combustion gases to be used in heating purposes such as space heating, residential water heating and to drive absorption chillers for cooling applications.

            Cogeneration technologies for residential, commercial and institutional applications can be classified according to their prime mover and from where their energy source is derived. Apart from reciprocating engine and micro-turbine based cogeneration systems for residential, commercial and institutional applications, technologies most likely to be successful long term are fuel cell based cogeneration systems and Sterling engine cogeneration systems because of their potential to achieve high efficiency and low emission levels.

Reciprocating Internal Combustion (IC) Engine Based

Cogeneration Systems

             Reciprocating engine based cogeneration systems are the prime mover of choice for small scale cogeneration applications16, providing electricity and thermal energy through heat recovery from the exhaust gas, engine oil and cooling water. This is attributed to their well-proven technology, robust nature, and reliability. However, they do need regular maintenance and servicing to ensure availability. They are available over a wide range of sizes ranging from a few kilowatts to more than ten megawatts, and can be fired on a broad variety of fuels with excellent availability, making them suitable for numerous cogeneration applications in residential, commercial, institutional and smallscale industrial loads.

             Reciprocating IC engines are based on the Otto cycle (spark ignition) or the Diesel cycle (compression ignition). In the Otto engine, the mixture of air and fuel is compressed in each cylinder before ignition is caused by an externally supplied spark. The Diesel engine involves only the compression of air in the cylinder and the fuel is introduced into the cylinder towards the end of the compression stroke, thus the spontaneous ignition is caused by the high temperature of the compressed mixture.

Reciprocating IC engines used for residential cogeneration applications of less than 30 kW are frequently based on spark ignition engines. The mechanical power derived from the engine turns the generator to produce electrical power; the heat from hot exhaust gases, cooling water and engine oil is harnessed to meet the thermal requirement of the building.

Principle of Operation

             Reciprocating internal combustion engines are classified by their method of ignition: compression ignition (Diesel) engines and spark ignition (Otto) engines. Diesel engines are primarily used for large-scale cogeneration, although they can also be used for small-scale cogeneration. These engines are mainly four-stroke direct injection engines fitted with a turbo-charger and intercooler. Diesel engines run on diesel fuel or heavy oil, or they can be set up to operate on a dual fuel mode that burns primarily natural gas with a small amount of diesel pilot fuel. Stationary diesel engines run at speeds between 500 and 1500 rpm. Cooling systems for diesel engines are more complex in comparison to the cooling systems of spark ignition engines and temperature are often lower, usually 85 C maximum, thus limiting the heat recovery potential

compared to Diesel engines, spark ignition (SI) engines are more suitable for smaller 

             Cogeneration applications, with their heat recovery system producing up to 160oC hot water or 20bar steam output. In cogeneration applications, spark ignition engines are mostly run on natural gas, although they can be set up to run on propane, gasoline or landfill gas. SI engines suitable for small cogeneration applications (e.g. residential) are open chamber engines. Many SI engines derived from Diesel engines (i.e. they use the same engine block, crankshaft, main bearings, camshaft, and connecting rods as the diesel engine) operate at lower brake mean effective pressure (BMEP) and peak pressure levels to prevent knock. Consequently, because of the derating effects of lower BMEP, the SI versions of Diesel engines usually produce 60-80% of the power output of the parent Diesel.

             Currently, the emission profile of natural gas fired SI engines has improved significantly through better design and control of the combustion process and through the use of exhaust catalysts. In addition, natural gas fired SI engines offer low first cost, fast start up, and significant heat recovery potential.




Typical packaged internal combustion engine based (spark ignited) cogeneration system

            The basic elements of a reciprocating internal combustion engine based cogeneration system are the engine, generator, heat recovery system, exhaust system, controls and acoustic enclosure. The generator is driven by the engine, and the useful heat is recovered from the engine exhaust and cooling systems. The architecture of a typical packaged internal combustion engine based cogeneration system is shown in Figure


            The engines used in cogeneration systems are lean/stoichiometric mixture engines since they have lower emission levels, and the excess oxygen in the exhaust gases can be used for supplementary firing. However, in lean burn engines, the increased exhaust gas flow causes a temperature decrease, resulting in lower heat recovery from the exhaust boiler.

            In most cogeneration systems, the engine is cooled using a pump driven forced circulation cooling system that forces a coolant through the engine passages and the heat exchanger to produce hot water.

            Natural cooling systems cool the engine by natural circulation of a boiling coolant through the engine, producing low-pressure saturated steam from the engine jacket.


Heat recovery

            Not all of the heat produced in an internal combustion engine based cogeneration system can be captured in on-site electric generation, because some of the heat energy is lost as low temperature heat within the exhaust gases, and as radiation and convection losses from the engine and generator.

            There are four sources where usable waste heat can be derived from a reciprocating internal combustion based cogeneration system: exhaust gas, engine jacket cooling water, and with smaller amounts of heat recovery, lube oil cooling water and turbocharger cooling. Heat from the engine jacket cooling water accounts for up to 30% of the energy input while the heat recovered from the engine exhaust represents 30 to 50%. Thus, by recovering heat from the cooling systems and exhaust, approximately 70-80% of the energy derived from the fuel is utilized to produce both electricity and useful heat as shown in Table

                                       Internal combustion engine co-generation process

                                                       Without Heat Recovery                  With Heat Recovery

Engine output at Flywheel                            35%                                                35%

Un-Recoverable heat                                     65%                                                21%

Recoverable heat                                            0%                                                 44%

Total useful energy                                      35%                                                 79%


(Values in bold represent useful energy)


            Heat recoveries from reciprocating internal combustion engine based cogeneration systems cannot be made directly to a building’s heating medium because of problems associated with pressure, corrosion, and thermal shock. Therefore, shell and tube heat exchangers or plate heat exchangers are used to transfer heat from the engine cooling medium to the building’s heating medium. Condensing heat exchangers can be employed to recover the latent heat that would otherwise be lost, however, they are suitable only with natural gas fired systems because of corrosion problems associated with other fossil fuels



             The efficiency of a cogeneration system is measured as the fraction of the input fuel that can usefully be recovered as power and heat. The remaining energy is lost as low temperature heat within the exhaust gases and as radiation and convention losses from the engine and generator. Water is produced as a combustion product when hydrocarbon fuel is burnt in the presence of oxygen, and the water is vaporized to steam by the heat of reaction. The efficiency is generally expressed in terms of both electrical efficiency and overall efficiency:


                                                  Electric Output ( kW)

Electrical efficiency       =     

                                                   Fuel Input (kW)

                                                   thermal output (kW)

Thermal efficiency           =

                                                       fuel input( kW)

                                                useful thermal +electrical output (kW)

Overall efficiency            =  

                                                                    fuel input kW





            The overall efficiency of a cogeneration system depends on the type of the prime mover, its size, and the temperature at which the recovered heat can be utilized. Also, the efficiency depends on the condition and operating regime of the cogeneration unit. The overall efficiency is however a first law efficiency that does not represent the quality of the electrical and heat production i.e. was the heat and electricity produced usefully used. For cogeneration systems it is worth considering the energy efficiency of the system, i.e. the availability or capacity of the system to perform useful work.


4) Hydronic System


            The science of heating and cooling with water is known as Hydronic. Through years of advancement in technology, it is believed that water is still the most practical, economical, and ecologically safe heat transfer medium.     


    Air Control in Hydronic System

            One problem that continues to crop up frequently in hydronic system, unless handled properly is the quantity of air permitted to circulate or clog a system. Air control system that are properly designed and install can eliminate major problems, reduce maintenance, cut cost of operation and perform efficiently.

           Effective air control will also prolong the life of the system and reduce unnecessary noise. Depending upon the type of system used, the admission of air to the system will vary.

     Once through

            A once through system passes water through the equipment only once, then discharges it to a sewer. 

    Recirculating system

            In a recirculating system, water is not discharged, but flows in a repeating circuit from the heat exchanger to the refrigeration equipment and back to the heat exchanger.



Components of Hydronic System

            Warmed water heating and chilled water cooling systems are at there best level of performance when the terminal equipment and piping circulates is positive and balanced, free of air, and systems are under proper pressure.

            The devices that performs or promote these functions are called “specialties” and

includes Air separator, Air vents, Flow valves, pressure relief valve, pressure reducing valve, radiator valve, balancing fittings, vent tees, and diverter fittings.


Water System Piping Classification.

              An important contribution to satisfaction performance is the method of piping. Water System Piping can be divided in two classifications.

I)       Circuit for small system

II)     Main distribution piping  

I) Circuit for small system

              These are the pipes circuits suitable for complete small systems or as terminal or branch circuits on large systems

These systems are classified as

1) Series loop system

              A series loop system is a contentious length of pipe or tube from a boiler or chiller supply connection back to the boiler or chiller return connection     

2) One Pipe System 

              One Pipe System makes use of single loop as a supply and return main. For each  terminal unit , a supply and a return tee are installed on the same main.

3) Two Pipe System

               Two Pipe Circuits may be direct – return or reverse- return. In direct –return ,the return main flow direction is opposite supply main flow; return water from each unit takes the shortest path.





II) Main distribution piping  

                 It is used to convey water to and from the terminal units or circuits in large system. It is further divided into fallowing types.

1) Three Pipe System

                 A Three Pipe System is usually used with an induction system and will satisfy the variation in load by providing independent source of heating and cooling to the room unit in the form of constant temperature primary or secondary chilled and hot water.

2) Four Pipe System.

                 It is having four pipes to each terminal unit. The four pipes consist of a cold water supply, a cold water return, a warm water supply, and a warm water return. Four Pipe System satisfies variation in cooling and heating to the induction unit using temperature primary air, secondary chilled water and secondary hot water.


Maintenance of HVAC Systems.

Research has shown that moisture incursion into debris-laden ductwork, coupled with a higher-than-normal level of carbon dioxide, a lower level of oxygen, and temperatures within the human comfort zone, create an ideal
environment for microbes to flourish. Such biological growth in the ductwork could also lead to populations of health-threatening microorganisms therein, including bacteria, viruses, yeasts and fungi. A recently publicized example is the incidence of Legionnaire's Disease which was linked to microorganism growth in a building's HVAC cooling tower. Large populations of allergy-causing molds and spores are also found in dirty ductwork. Incidents of allergies and chronic respiratory problems experienced by many workers in large buildings are thought to be caused by recirculating air kept in a contaminated state by passing through contaminated ducts. Hence, there is a need for a method for effectively cleaning debris and biological growth from HVAC ductwork, especially in large or tall buildings. There is also a need for such a method that can be regularly and conveniently performed With minimal disruption of normal activity in the building. Further, there is a need for a method that would allow convenient disinfection of the HVAC ductwork before and/or after such cleaning, the disinfection being performed using chemicals that do not produce obnoxious odors or cause eye or respiratory irritation in nearby personnel. Further, there is a need for such a method that does not require cutting large openings in the ductwork or the destructive opening of walls and panels in order to gain access to the ductwork.


            The present invention is a method and apparatus for cleaning and disinfecting HVAC ductwork, particularly large and complex ductwork systems. The method is simple and convenient to perform and causes minimal
disruption of work activity in the building. Further, the ductwork can be cleaned and disinfected without the need for dismantling either the
ductwork or other structures such as walls or floors.
          To assess the degree of debris accumulation in the ductwork, a fiber-optics borescope may be used to visually examine the entire interior of the ductwork. Before commencing the cleaning steps, the interior of the ductwork, including debris accumulated therein, may be disinfected using a disinfectant aerosol injected into the ductwork. Such disinfection reduces the potential health hazard associated with dislodging and collecting fine debris and flock possibly containing pathogenic microorganisms.
           The cleaning method basically includes steps in which the debris is agitated and dislodged from the interior surfaces of the ductwork using an impinging high velocity air stream from a nozzle-ended hose supplied by a portable air compressor. The hose is inserted through an opening in the duct wall and slowly urged to move downstream through the duct as debris is dislodged. During agitation of the nozzle-ended hose, a worker observes the cleaning process through a rigid borescope inserted through the same duct opening as the hose. When cleaning a return duct, debris-laden air is moved out of there turn duct and into a filter by operation of the HVAC unit blower, a particle filter being installed between the return duct and the HVAC unit. To move debris-laden air out of a supply duct and filter the debris from the air, the HVAC unit is likewise operated and, in addition, a portable blower/filter unit is connected to the distal end of the supply duct being cleaned. By employing the HVAC unit and the portable blower/filter unit to clear debris-laden air out of the supply duct, the debris is cleaned out of the entire ductwork using the normal direction of air flow through the ducts.
            The nozzle at the end of the air hose is designed so that, when high pressure air is supplied to the nozzle, the hose whips around causing the nozzle to beat off debris without striking the interior walls of the ducts. In particular, the nozzle has a plurality of radial passageways which terminate in orifices that direct jets of air back along the hose at an acute angle to the hose axis. Control of nozzle movement is maintained by adjusting a hand-held regulator that is located between the nozzle and the air source. Because the method of the present invention employs equipment that is portable and transportable in elevators, it is possible for the first time to regularly clean and disinfect the HVAC ductwork and associated equipment in tall buildings, particularly buildings taller than two to three stories.

A primary object of the present invention is to provide a method of cleaning and disinfecting a complex HVAC ductwork, particularly as found in large or tall buildings, without having to dismantle the ductwork or other structures.

Another object is to provide a method for cleaning and disinfecting such HVAC ductwork without causing significant disruption of normal activity in the building.
           Another object is to provide a method for examining the entire ductwork during cleaning without having to cut large inspection ports in the ductwork.