System load is measured in either BTUs or pounds of steam (at a specific pressure and temperature). It would be nearly impossible to size and select a boiler(s) without knowing the system load requirements. Knowing the requirements leads to the following information:
- The boiler(s) capacity, taken from the maximum system load requirement.
- The boiler(s) turndown, taken from the minimum system load requirement.
- Conditions for maximum efficiency, taken from the average system load requirement.
Determining the total system load requires an understanding of the type(s) of load in the system. There are three types of loads: heating, process, and combination.
A heating load is typically low-pressure steam or hot water, and is relatively simple to define because there is not a great deal of instantaneous change to the load. A heating load is used to maintain building heat. Characteristics of a heating load include large seasonal variations but small instantaneous demand changes. The boiler should be sized to accommodate the worst possible weather conditions, but being mindful that the extreme condition is rarely reached. Excessive cycling because of an oversized boiler relative to load is extremely inefficient. Many boilers operate more than 90% of the time at loads which are a fraction of their rated capacity. Unless the sizing and selection of the boiler takes these seasonal variations into consideration, excessive cycling and fuel loss will occur.
A process load is usually a high-pressure steam load. A process load pertains to manufacturing operations, where heat from steam or hot water is used in the process. A process load is further defined as either continuous or batch. In a continuous load, the demand is fairly constant - such as in a heating load. The batch load is characterized by short-term demands. The batch load is a key issue when selecting equipment, because a batch-type process load can have a very large instantaneous demand that can be several times larger than the rating of the boiler. For example, based on its size, a heating coil can consume a large amount of steam simply to fill and pressurize the coil. When designing a boiler room for a process load with instantaneous demand, a more careful boiler selection process should take place.
Many facilities have a mixture of loads - different types of process loads and combinations of heating and process loads. The information just given on heating and process loads should be taken into consideration when dealing with a combination load.
Defining 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.
- 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.
- 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.
- 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.
Load tracking is the ability of a boiler to respond to changes in steam or hot water demand. Most often associated with process loads, load tracking focuses on the boiler's ability to supply a constant volume of steam at the required pressure.
The ability of the boiler to track a variable load depends on the boiler type, burner turndown capability, feedwater valve control, and combustion control design. If the analysis of the load shows highly variable load conditions, a more complex control package may be necessary. This type of control is achieved with sophisticated boiler management systems. For more information on these types of systems, contact your local Cleaver-Brooks authorized representative.
If the application has instantaneous load demands, whereby a large volume of steam is required for a short period of time, a boiler with a large energy storage reserve, such as a firetube, should be considered. If the application dictates large variances in load demand, where the load swings frequently for long periods of time, the best choice is probably a watertube boiler, because it contains less water and can respond to the variances more rapidly.
In all cases, operation of the burner should be taken into account in selecting a boiler(s) to meet system demand. The burner will require proper operating controls that can accurately sense the varying demands and be capable of the turndown requirements. The boiler feedwater valve and control design are also critical if load swings are expected.