Now, it is known fact that when the gas velocity exceeds the sound-velocity, the heat is transferred from the gas at a much higher rate than rates achieved with the sub-sonic flow. The advantages of this theory are taken to effect the large heat transfer from a smaller surface area in this boiler.
Air is compressed to 2.5 bar with a help of a compressor run by gas turbine before supplying to the combustion chamber to get the supersonic velocity of the gases passing through the combustion chamber and gas tubes and high heat release rates (40 MW/m3). The burned gases in the combustion chamber are passed through the annulus of the tubes. The heat is transferred from gases to water while passing through the annulus to generate the steam. The mixture of water and steam thus formed then passes into a separator which is so designed that the mixture enters with a spiral flow. The centrifugal force thus produced causes the heavier water particles to be thrown outward on the walls. This effect separates the steam from water. The separated steam is further passed to the superheater and then supplied to the prime-mover. The water removed from the steam in the separator is again passed into the water tubes with the help of a pump.
The gases coming out from the annulus at the top are further passed over the superheater where its heat is used-for superheating the steam. The gases coming out of superheater are used to run a gas turbine as they carry sufficient kinetic energy. The power output of the gas turbine is used to run the air compressor. The exhaust gases coming out from the gas turbine are passed through the economizer to utilize the remaining heat of the gases. The extra power required to run the compressor is supplied with the help of an electric motor. Feedwater of 10 to 20 times the weight of steam generated is circulated through the tubes with the help of the water circulating pump. This prevents the overheating of metal walls.
The size of the Velox boiler is limited to 100 tons per hour because 400 KW is required to run the air compressor at this output. The power developed by the gas turbine is not sufficient to run the compressor and therefore some power from an external source must be supplied as mentioned above.
Advantages:
1. Very high combustion rates are possible as 40 MJ/m3 of combustion chamber volume.
2. Low excess air is required as the pressurized air is used and the problem of drought is simplified.
3. It is a very compact generating unit and has greater flexibility.
4. It can be quickly started even though the separator has a storage capacity of about 10% of the maximum hourly output.
