How to define load variations?

Loads vary, and a power plant must be capable of handling the minimum load, the maximum load, and any load variations. Boiler selection is often dictated by the variation in load demand, rather than by the total quantity of steam or hot water required. There are three basic types of load variations: seasonal, daily, and instantaneous.

Daily Variation. Daily variation can occur due to variations in the work hours, or the heat required at various times of the day or weekend. Minimum and maximum seasonal variations mentioned earlier may already reflect these changes if they occur daily. If not, the minimum and maximum daily loads should be included.

Seasonal Variations. For a heating system, seasonal variations can mean no demand in the summer, light demand in the fall and spring, and heavy demand in the winter. Manufacturing operations often have seasonal variations, because the demand for production may vary. When selecting boiler equipment, the minimum and maximum load for each season should be determined.

The seasonal and daily variations define the size of the load that the boiler(s) must handle. Seasonal and daily variations also help define the number of boilers and turndown requirements.

Instantaneous Demand. Instantaneous demand is a sudden peak load change that is usually of short duration. These types of loads are sometimes hidden. Many machines or processes are rated in pounds of steam per hour or BTU/hr as running loads, under balanced operating conditions, and there is no recognition given to "cold startup," "peak," or "pickup” loads. Heating loads such as using boiler water to heat domestic hot water can be another example of a sudden peak load for a short duration. The instantaneous load demand is important to consider when selecting a boiler to ensure that these load variations are taken into account. If the instantaneous demand is not included in the system load calculations, the boiler(s) may be undersized.