Correctly Sizing a Steam Boiler-What do I need to know?
The correct selection of a steam boiler output is critical to its operation, efficiency and performance. If undersized for the system load, then steam pressure will rapidly decrease thus causing the boiler to collapse its steam space, in turn causing low water alarms and ultimately burner lockout.
To prevent this, the boiler needs to be matched correctly to the system steady state demand and any short-term peak demands that may occur. In short, ensure that the boiler is sized to the system’s worst case scenario!
To achieve optimum system steam supply to demand ratio, it is important to know what you’re trying to achieve. For example what you are heating, its mass and the temperature rise you require, over which timescales (i.e. how quickly do you need to heat your product?)
The equation to determine this is:
Q = W x Sp x t
L x h
Where:
Q = mass flowrate of steam required.................................kg/h or lb/h
W = mass of substance heated.........................................kg or lb
Sp= specific heat capacity of the substance being heated
(Water)...............................................................................4.186kJ/kgºC
t = Temperature rise..........................................................ºC
L = Latent Heat of evaporation (hfg) of steam at 7 barg ..2048 kJ/kg
h = Recovery time..............................................................hours
Example:
A tank containing 2500 litres of water that rises in temperature from ambient 10ºC to 70ºC in 2 hours.
2500 x 4.186 x 60
2048 x 2
= 154kg/h of steam
This figure represents the actual steam energy required to heat the 2500 litres. You will also need to consider the extra energy required to heat the tank. If it’s steel, you need to carry out the same calculation using the mass of the tank multiplied by 0.5 (this the specific heat of steel, multiplied by 60ºC rise, divided by the latent heat).
To determine the boiler rating to achieve this actual steam load, consideration needs to be taken into account of the “F&A 100ºC” rating. All steam boiler manufacturers use this rating because they must take the standard assumption that water evaporates at 100ºC and that feed water is 100ºC.
The actual evaporation temperature of water at higher pressures is higher than 100ºC and the amount of condensate return will affect the feed water temperature.
For example, the evaporation temperature of water from a boiler operating at 7 barg is 170ºC and a low condensate return level could see feed water as low as 30ºC. At this point you will require steam to heat the feed tank to 85 to 90ºC.
Typically, you need to add 30 to 35% to the calculated system load to take into account the F&A rating and general system losses from pipework and blowdown.
With this in mind, giving a system load of 154kg/h, you will require a boiler rating of 208kg/h F&A 100ºC. That’s equivalent to a Fulton 15J vertical steam boiler!