Air-cooled condenser types

There are two types under this category, viz.

 (a) natural convection and

(b) forced air type.

 

 

Natural Convection Condenser

Air movement over the surface of condenser tubes is by natural convection. As air comes in contact with the warm-condenser tubes, it absorbs heat from the refrigerant and thus the temperature of the air increases. Warm air being lighter, rises up and in its place cooler air from below rises to take away the heat from the condenser. This cycle goes on. Since air moves very slowly by natural convection, the rate of flow of heat from the refrigerant to air will be small. Thus a natural convection condenser is not capable of rejecting heat rapidly. Therefore a relati vely large surface area of the condenser is required. Hence the use of this type of condenser is limited to very small units such as domestic refrigerators. It, however, requires very 45 Refrigeration Equipment little maintenance.

 

In the small units, the condenser is fixed at the rear of the refrigerator cabinets. Generally, steel tubes are used, steel being cheaper than copper. To increase the heat-transfer area, wires are welded to the condenser tubes.These wires provide mechanical strength to the coil as well. In certain designs,widely-spaced fins are used. It is necessary to space the fins quite widely to avoid resistance to free (natural convection) air movement over the condenser. Still another design is the plate-type. The condenser coil is fastened to aplate. The plate being in contact with the condenser tubes, the surface area of the condenser is increased. The plate-type condenser is mounted on the back of the refrigerator cabinet with a small gap between the cabinet and the plate. This gap gives an air-flue effect and facilitates better natural convection air currents.It is obvious that while locating refrigerators or deep-freezes cabinets with a natural convection condenser fixed on the cabinet, sufficient care should be taken to allow free air movement. Also they should not be near an oven or

any warm location.

 

Forced-air Circulation Condenser

 

This type employs a fan or blower to move air over the condenser coil at a certain velocity. The condenser coil is of the finned type. Fins in such coils are closely spaced (ranging between 8 and 17 fins per inch). The space between the fins gets choked with dirt and lint. Therefore to obtain optimum capacity, the fins should be kept clean. For circulating air over the condenser, fans are mounted on the shaft/pulley of the compressor motor. 

 

For bigger-capacity plants a separate motor is used to drive the fan or blower as also for hermetic-compressor units

 

 

 

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Condenser

The functions of the condenser are to desuperheat the high pressure gas, condense
it and also sub-cool the liquid.
Heat from the hot refrigerant gas is rejected in the condenser to the condensing
medium-air or water. Air and water are chosen because they are naturally
available. Their normal temperature range is satisfactory for condensing
refrigerants.
Like the evaporator, the condenser is also heat exchange equipment

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Atmospheric Fluidized Bed Combustion (AFBC) Boiler

Atmospheric Fluidized Bed Combustion (AFBC) Boiler

Most operational boiler of this type is of the Atmospheric Fluidized Bed Combustion. (AFBC). This involves little more than adding a fluidized bed combustor to a conventional shell boiler. Such systems have similarly being installed in conjunction with conventional water tube boiler.

Coal is crushed to a size of 1 – 10 mm depending on the rank of coal, type of fuel fed to the combustion chamber. The atmospheric air, which acts as both the fluidization and combustion air, is delivered at a pressure, after being preheated by the exhaust fuel gases. The in-bed tubes carrying water generally act as the evaporator. The gaseous products of combustion pass over the super heater sections of the boiler flow past the economizer, the dust collectors and the air preheater before being exhausted to atmosphere.

Pressurized Fluidized Bed Combustion (PFBC) Boiler

Pressurized Fluidized Bed Combustion (PFBC) Boiler

In Pressurized Fluidized Bed Combustion (PFBC) type, a compressor supplies the Forced Draft (FD) air and the combustor is a pressure vessel. The heat release rate in the bed is proportional to the bed pressure and hence a deep bed is used to extract large amount of heat. This will improve the combustion efficiency and sulphur dioxide absorption in the bed. The steam is generated in the two tube bundles, one in the bed and one above it. Hot flue gases drive a power generating gas turbine. The PFBC system can be used for cogeneration (steam and electricity) or combined cycle power
generation. The combined cycle operation (gas turbine &
steam turbine) improves the overall conversion efficiency by
 5 to 8%.

Atmospheric Circulating Fluidized Bed Combustion Boilers (CFBC)

Atmospheric Circulating Fluidized Bed Combustion Boilers (CFBC)

In a circulating system the bed parameters are so maintained as to promote solids elutriation from the bed. They are lifted in a relatively dilute phase in a solids riser, and a down-comer with a cyclone provides a return path for the solids. There are no steam generation tubes immersed in the bed. Generation and super heating of steam takes place in the convection section, water walls, at the exit of the riser.

CFBC boilers are generally more economical than AFBC boilers for industrial application requiring more than 75 – 100 T/hr of steam. For large units, the taller furnace characteristics of CFBC boilers offers better space utilization, greater fuel particle and sorbent residence time for efficient combustion and SO2 capture, and easier application of staged combustion techniques for NOx control than AFBC steam generators.

Stoker Fired Boilers

Stoker Fired Boilers

Stokers are classified according to the method of feeding fuel to the furnace and by the type of grate. The main classifications are spreader stoker and chain-gate or traveling-gate stoker.

Spreader Stokers

Spreader stokers utilize a combination of suspension burning and grate burning. The coal is continually fed into the furnace above a burning bed of coal. The coal fines are burned in suspension; the larger particles fall to the grate, where they are burned in a thin, fast-burning coal bed. This method of firing provides good flexibility to meet load fluctuations, since ignition is almost instantaneous when firing rate is increased. Due to this, the spreader stoker is favored over other types of stokers in many industrial applications.

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Chain-grate or Traveling-grate Stoker

Chain-grate or Traveling-grate Stoker

Coal is fed onto one end of a moving steel grate. As grate moves along the length of the furnace, the coal burns before dropping off at the end as ash. Some degree of skill is required, particularly when setting up the grate, air dampers and baffles, to ensure clean combustion leaving the minimum of unburnt carbon in the ash.

The coal-feed hopper runs along the entire coal-feed end of the furnace. A coal gate is used to control the rate at which coal is fed into the furnace by controlling the thickness of the fuel bed. Coal must be uniform in size as large lumps will not burn out completely by the time they reach the end of the grate.

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Waste Heat Boiler

Waste Heat Boiler

Wherever the waste heat is available at medium or high temperatures, a waste heat boiler can be installed economically. Wherever the steam demand is more than the steam generated during waste heat, auxiliary fuel burners are also used. If there is no direct use of steam, the steam may be let down in a steam turbine-generator set and power produced from it. It is widely used in the heat recovery from exhaust gases from gas turbines and diesel engines.




                          

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Pulverized Fuel Boiler

Pulverized Fuel Boiler

Most coal-fired power station boilers use pulverized coal, and many of the larger industrial water-tube boilers also use this pulverized fuel. This technology is well developed, and there are thousands of units around the world, accounting for well over 90% of coal-fired capacity.

The coal is ground (pulverized) to a fine powder, so that less than 2% is +300 micro meter (μm) and 70-75% is below 75 microns, for a bituminous coal. It should be noted that too fine a powder is wasteful of grinding mill power. On the other hand, too coarse a powder does not burn completely in the combustion chamber and results in higher unburnt losses.
The pulverized coal is blown with part of the combustion air into the boiler plant through a series of burner nozzles. Secondary and tertiary air may also be added. Combustion takes place at temperatures from 1300-1700°C, depending largely on coal grade. Particle residence time in the boiler is typically 2 to 5 seconds, and the particles must be small enough for complete combustion to have taken place during this time.

This system has many advantages such as ability to fire varying quality of coal, quick responses to changes in load, use of high pre-heat air temperatures etc.

One of the most popular systems for firing pulverized coal is the tangential firing using four burners corner to corner to create a fireball at the center of the furnace.

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Thermic Fluid Heater

Thermic Fluid Heater

In recent times, thermic fluid heaters have found wide application for indirect process heating. Employing petroleum - based fluids as the heat transfer medium, these heaters provide constantly maintainable temperatures for the user equipment. The combustion system comprises of a fixed grate with mechanical draft arrangements.

The modern oil fired thermic fluid heater consists of a double coil, three pass construction and fitted with modulated pressure jet system. The thermic fluid, which acts as a heat carrier, is heated up in the heater and circulated through the user equipment. There it transfers heat for the process through a heat exchanger and the fluid is then returned to the heater. The flow of thermic fluid at the user end is controlled by a pneumatically operated control valve, based on the operating temperature. The heater operates on low or high fire depending on the return oil temperature, which varies with the system load

The advantages of these heaters are:

• Closed cycle operation with minimum losses as compared to steam boilers.
• Non-Pressurized system operation even for temperatures around 250 0c as against 40 kg/cm2 steam pressure requirement in a similar steam system.
• Automatic control settings, which offer operational flexibility.
• Good thermal efficiencies as losses due to blow down, condensate drain and flash steam do not exist in a thermic fluid heater system.

The overall economics of the thermic fluid heater will depend upon the specific application and reference basis. Coal fired thermic fluid heaters with a thermal efficiency range of 55-65% may compare favorably with most boilers. Incorporation of heat recovery devices in the flue gas path enhances the thermal efficiency levels further.

 

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