Air, water and steam are three media commonly used to distribute heat to process loads. However, steam has several advantages compared to hot air and hot water. These advantages include.
- the heat carrying capacity of steam is much greater than air or water
- steam provides its own locomotive force.
- steam provides heat at a constant temperature
How much more efficient is steam compared to hot air? If 100 psig steam were condensed in a heat exchanger, the mass flow rate of steam required to transfer 1,000,000Btu/hr of heat would be about 1,135 lb/hr.
If the temperature of hot air dropped by 100 F as it passed through a heat exchanger, the mass flow rate of air to transfer the same amount of heat with the same temperature difference would be about ________?
And the answer is…
38,500 lb/hr or about 34 times as much as steam. Here are the calculations to see for yourself:
- Msteam = Q / hfg = 1,000,000 Btu/hr / 881 Btu/lb = 1,135 lb/hr
- Mwater = Q / (cp x dt) = 1,000,000 Btu/hr / (1 Btu/lb-F x 100 F) = 10,000 lb/hr
Mair = Q / (cp x dt) = 1,000,000 Btu/hr / (0.26 Btu/lb-F x 100 F) = 38,500 lb/hr
The higher flow rates required by water and air require pipes and ducts with larger diameters than steam pipes, which increases first cost and heat loss. In addition, air and water do not propel themselves. Thus, hot air and water distribution systems require fans or pumps, whereas a steam distribution system does not require any additional propulsion for outgoing steam and a very small pumping system for returning the condensate to the boiler.
Finally, because steam condenses at a constant temperature, 100-psig steam could heat a process stream to a maximum temperature of 338 F which is the temperature of the steam. On the other hand, the temperature of water and air decrease as heat is transferred; thus, if the heat in these examples was delivered by a cross-flow heat exchanger, the maximum temperature of the process stream would be 100 F less than the incoming temperature of the air or water.
Because of these advantages, steam is the most widely used heat-carrying medium in the world.