Velox Boiler is a high-pressure water tube boiler. Again if heat supplies through the tube and water supply around the tube then this boiler can be a fire tube boiler. Velox Boiler uses GAS in the Combustion Chamber. This Boiler has a unique basic theory than other boilers that we already discussed.
When the GAS velocity exceeds the velocity of sound, then the heat transfer rate form the GAS much higher than the rate achieve in subsonic flow. Velox Boiler used the advantage of the theory. By applying this theory Velox Boiler can increase the total heat transfer rate that means steam generation rate without actually increasing its size. So we can say, a large amount of heat will be transferred from GAS to water. For this special reason, Velox Boiler is one of the best bowlers in the Gas Turbine industries. This boiler has a thermal efficiency of about 90 to 95 percent.
Construction of Velox Boiler:
Velox Boiler is a forced circulation water tube boiler. Let’s start with the Feed Pump. This Feed Pump supply the water at a pressure and water pass through the Economizer like Loeffler Boiler. Hot fuel GAS passes through the Economizer from the Combustion Chamber. When water passes through the Economizer Tube then water is pre-heated. Then you will find another water circulating pump and this pump is connected with the Economizer.
This water circulating pump is connected with the Tube Evaporating Section. An Axial Compressor and a Steam Separator is connected with the Tube Evaporating Section. Axial Compressor usually is driven by a GAS Turbine. Then Steam Separator is connected with the Convection Super Heater where saturated steam from Steam Separator gets heated and becomes Superheated Steam. Velox Boiler uses Pressurized Combustion Chamber.
Working Principle of Velox Boiler:
At first Economizer Tube received feed water when Feed Water Pump starts to rotate. In the meantime, the GAS Turbine drives the Axial Compressor. This Axial Compressor increases the incoming air pressure from Atmospheric pressure to Furnace pressure. As we know Velox Boiler works as a basic heat exchanger. By this way more heat release by the fuel which increases the velocity of the flue GAS at the sound velocity level. After leaving the Economizer to feed water becomes heated and a Water Circulating Pump supplies this hot water to the Tube Evaporating Section.
In this Tube Evaporating Section, most of the water converted into steam. Then steam supply to the Steam Separating Section. From this section, steam supply to the Convection Super Heater. If there is any moisture with the steam then it will be separated in the Steam Separating Section and send it to the Tube Evaporating Section by Water Circulating Pump. In the Convection Super Heater steam temperature increases with the help of the exhaust flue GAS. Then generated superheated steam, supply to the prime mover for further operation.
The size of a Velox Boiler is limited to 100 tonnes per hour. The main reason is, 600 BHP is required to run the air compressor at 100 tonnes output. The amount of power generated by the GAS Turbine is not enough to run this compressor. For this reason, some external power source must be needed.
Advantage and Disadvantage of Velox Boiler:
Velox Boiler is compact in design with high flexibility. This boiler has a combustion rate and it can start quality. Velox Boiler needs low access air because this boiler uses pressurized air by a compressor. But there is a limitation of Velox Boiler for its size.