Steam is often used for heating liquids. Heat is provided to a liquid either through a heat exchanger or by direct injection of raw steam. The injection of steam directly into the process is known as steam sparging. The sparge design and location affect the efficiency of the process.
Keywords: boiling, heating water, temperature increase, injector, lance, boiler pressure, properties of steam.
PROPERTIES OF STEAM
As water is heated in a vessel it boils and some turns into a vapour called ‘wet’ steam. If heated further the water is all boiled away and at that point the vapour it is called saturated vapour. If the vapour is heated still more it becomes super heated steam. In super heated steam the water molecules are at very high energy levels.
Pressure also affects the amount of energy in steam. Water at sea level boils at 100 oC (212°F) while on top of Mount Everest it boils at a lower temperature, and in a pressure cooker heated on a stove it boils at a higher temperature. A higher pressure permits higher temperature and energy.
If water is to be used to make steam at more then 100 oC on planet Earth it is done in a pressure vessel called a boiler.
At a given pressure steam takes up a specific volume per kilogram. The lower the pressure the larger the volume needed for the same amount of steam. One kilogram of saturated steam at sea level atmospheric conditions will be at 100°C, 1 atmosphere pressure and require 1.7 cubic meters volume. The same kilogram at six times atmospheric pressure will be at 158°C and squeezed to a volume of 0.3 cubic meters. At 100 times atmospheric pressure it will be at 311°C and squeezed into a 0.018 cubic meter space.
If steam at 100 times atmospheric pressure were released at sea level it would expand instantly 95 times and loose 211°C in temperature. You would see and hear a massive plume of vapour streaming out of the hole at very high velocities. The excess heat is radiated into the air around the plume.
HEATING LIQUIDS WITH DIRECT STEAM INJECTION
Injecting steam directly into a liquid puts the molecules of high-energy steam in direct contact with the liquid molecules. The energy is transferred from the hotter to the colder molecules and so the process liquid warms-up. As more steam is injected the liquid’s temperature rises toward the steam’s temperature.
The temperature rise one kilogram of steam can cause to one kilogram of liquid depends on the ability of the liquid to take in the energy. This ability to absorb energy is known as the liquid’s specific energy. It is the energy needed to raise the temperature of one kilogram of the liquid by one degree centigrade. If we can find out how much specific energy is needed to heat one kilogram of a liquid one degree we can calculate how much steam, at a certain temperature and pressure, is needed to heat the liquid.
The time taken to heat the liquid depends on how fast the steam is introduced, how much hotter the steam is than the liquid and how well it is distributed through the liquid. If 1000 kilograms of liquid is to be heated 100 oC higher but only one kilogram of steam per hour could be supplied it will take an eternity to warm. Similarly if the steam was only 101 oC hotter than the liquid there would be an initial surge in the liquid temperature as it warmed but the final few degrees rise would take longer and longer. And unless the steam is evenly distributed in the liquid there would be pockets of hot liquid around the sparge, with the liquid further away getting progressively colder.
When heating liquids with a sparge insure there is a plentiful supply of steam at sufficiently high temperature and the steam and process liquid are well mixed together. Examples of some typical steam sparges are shown in Figure 1.
How fast the steam can be injected into the liquid depends on the steam pressure and the size of the hole through which the steam is squirted. With the specific energy of the process liquid and steam known it is only necessary to decide how quickly to heat up the liquid and then the size of the hole to provide the steam can be calculated.
Once the injection hole area is known, the size of the pipe work to supply the steam from the boiler can be determined. Usually a control valve responding to a temperature sensor in the process liquid is installed in the steam supply line. As the process temperature rises toward the required or ‘set’ temperature the control valve regulates the rate of steam supply and the speed of temperature rise. Figure 2 shows a steam sparging circuit for a boiler feed water tank.
CONNECTING SPARGES TO THE EQUIPMENT
Steam sparging can generate a great amount of vibration. Especially if there is a big pressure difference between the steam pressure and the process pressure. When pressure drops steam expands. In order for steam to flow through the pipe to the outlet at the same rate it is turning from high pressure to low pressure steam it speeds up. The increase in velocity causes vibration and if sufficiently fast it will gradually wear away the sides of the exit hole(s).
Steam sparging can generate a great amount of vibration. Especially if there is a big pressure difference between the steam pressure and the process pressure. When pressure drops steam expands. In order for steam to flow through the pipe to the outlet at the same rate it is turning from high pressure to low pressure steam it speeds up. The increase in velocity causes vibration and if sufficiently fast it will gradually wear away the sides of the exit hole(s).
Sparges welded into vessels need large, thick compensating plates to spread the vibration over a larger area of weld. If the weld area is small it cracks from work hardening caused by the vibrating sparge. This is important on stainless steel vessels as stainless has little resistance to work hardening.
Sparge lances put directly into the liquid require many holes below the liquid surface to quickly vent off the steam. Make enough 4 to 6 mm holes in the sparge so the sum of their areas is at least equal to one and a half times the area of the steam supply pipe.
Mike Sondalini – Maintenance Engineer
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I want to get a boiler for my home to ensure that I can get hot water. It makes sense that using the right steam methods would be important! I’ll be sure to work with a professional to ensure that I do all of this right.