Modular, on-demand boilers are not exactly new. They have been in use since the 1980s and have proven to be reliable, efficient alternatives to larger boiler systems. Yet, many engineers, designers and facility operators seem reluctant to adopt them despite three key benefits: high pressure output, low energy consumption and a space-saving, compact design.

Perhaps the reluctance to use modular boilers stems from the perceived lack of a strong track record, and from the conservative attitudes engineers are trained to cultivate. This is understandable. In the engineering profession, being conservative is being conscientious. A great deal rides on the work of engineers — not the least of which are public safety, environmental concerns and financial responsibility.

Yet, with the number of modular, on-demand boilers now in use — in nearly every type of industry and facility — wider acceptance makes sense. The technology is backed by a solid record in applications from breweries and distilleries, to chemical and food-processing plants, and to hospitals and laundry services.

Compared to conventional systems, modular on-demand boiler systems consume less energy while meeting steam demands — especially steam demands that vary over time. They do this by dividing output capacity among multiple small units while achieving good efficiency. For example, a conventional boiler is rated at 240,000 BTU/hr of energy lost through the boiler’s exterior. By comparison, energy losses with on-demand boilers are rated at 60,000 BTU/hr when fully operational.

While modular, on-demand boilers are suited to installations in which steam demand varies, they also are suited for other purposes. Features such as their compact, space-saving design, fuel efficiency and cost savings provide value in many environments. Typically, a modular boiler requires half the space of a conventional boiler, and it can generate the same amount of boiler horsepower (BHP) in 50 to 60 percent of the space required by larger boiler systems.

In addition, facilities with modular boilers can save (on average) 10 percent in energy costs. Modular systems may help extend component life because the individual boilers can be alternated during periods of lower loads, reducing wear.

Quick Response

Modular, on-demand boilers are well suited for situations that require a fast response to changing steam-load demands. For example, imagine a facility with wildly varying boiler horsepower demand: 50 horsepower at 8 a.m., 300 horsepower at 8:15 a.m., 1,000 horsepower at 2 p.m., then 50 horsepower again in the evening. While a traditional boiler might struggle to keep up with the changing demands, the modular boilers are design to respond quickly. The once-through vertical-tube design allows them to go from room temperature to full production in less than five minutes. (By comparison, conventional boilers typically have a 60- to 90-minute startup.) The power-up, power-down capability allows the modular boilers to respond quickly.

Like instantaneous hot-water heaters, the on-demand modular boilers are low-water-content designs. Almost by definition, they respond quickly to demand.

Design Decisions

When designing a boiler system for a specific facility, many engineering decisions are made that have long-lasting consequences. When designing systems to meet high steam demands, if a large boiler system is specified, the designer may feel he has one shot to get it right. Moreover, the facility will have to “live with” the design decisions for a long time. So, the engineer may be inclined to design for the worst-case scenario: one in which every piece of equipment is at maximum load at the same moment.

Imagine this: An engineer calculates that 779 horsepower is needed. The inclination then is to round that number up to 800 horsepower, “just in case.” Then, the engineer may wonder, what if my calculations were off a little bit — despite my best efforts? What if the equipment has a higher radiation? What if they want to increase their process capacity by 5 percent? Then, the inclination is to specify perhaps 900 horsepower.

Next imagine: A supplier offers a great deal on a 1,000-horsepower system that is available immediately. Why not do that?

In trying to plan for anything that might happen, suddenly the specification has grown from 779 horsepower to 1,000 horsepower. The effort to reduce the risk of underdesigning may have resulted in an oversized system. The plant will deal with the consequences in terms of energy efficiency and system responsiveness for perhaps 30 years.

It also is important to understand that the average steam load, when operating, is typically only 30 to 40 percent on a system that is right-sized. If the boiler is oversized, the average load becomes lower for that system. Even with a burner that can turn down to 10 percent of the maximum capacity, the boiler operates less efficiently during the periods below the average load.

A modular system approach allows the engineer to design a smaller system up-front, leaving space in the facility design for additional modular boilers if needed in the future. So, for the same process requiring 779 horsepower, perhaps three 200-horsepower modular units could handle the load 90 percent of the time. If the engineer designs in one backup modular boiler to cover contingencies on demand, the capacity exists without energy consumption unless demand warrants it. Moreover, redundancy is built into the system. If space is left in the facility design for future expansion, any currently unforeseen need could be met by adding more modular units.

In sizing a modular, on-demand boiler system it is important to know three values:

  • Maximum demand.
  • Minimum demand.
  • Average demand.

The smallest individual module should be sized to meet the minimum demand. Some modular systems may need the same module sizes for best performance; however, boiler control technology can allow differently sized modules to be controlled easily.

Evolving Technology

Recent developments in modular boiler design include communication systems that can control a series of modular boilers. These systems permit modular systems to, in effect, function as a single, efficient unit. The output of the boilers can be automatically adjusted — according to the need — within seconds.

The communication system is managed by a controller that receives signals from a sensor monitoring steam demand. Based on the sensor inputs, the boiler control system bring boilers online, regulates their output and shuts them down. The master controller’s software sends commands to the individual terminals to adjust the boilers’ operation for maximum overall efficiency.

Many modular boilers offer efficiency improvements thanks to the addition of variable-speed drives (VSDs). The VSDs slow the fan speed in low load situations and increase it when more air or fuel is needed. This conserves electricity and helps reduce the cost of operation. Modular boilers are able to achieve a fuel-to-steam efficiency rate of 85 percent.

While they are rather complex devices, modular boilers offer ease of use. This feature — combined with low maintenance requirements — simplifies operation operate on a day-to-day basis.

In conclusion, modular, on-demand boilers offer many benefits. In today’s world of environmental concerns, pressures to reduce costs and the need to do more with less, they indeed seem made to order.