Case study

The analized case requires an average dry saturated steam flow rate of 8 t/h at 12 bar, 14 hours a day, 6 out of 7 days.

Table 1 shows the project data used to determine the typical curves of the generators being studied.

The thermal demand in the case being studied would be met considering three types of modern steam generators:

• Extremely highly energy efficient steam generator with HE Smart technology;
• Steam generator with economiser (ECO), a highly efficient solution conventionally offered on the market;
• “Base” version boiler with no heat recovery system.

The performance of the boilers being compared was calculated using experimental in-house numerical software.

Source: Cannon Bono Energia S.p.A software.

HE Smart technology stands out for its simultaneous, instantaneous, self-adaptive, and integrated combination and management of the various technologies recovering heat from the hot flue gas (economiser and pre-heater for the air supplying the combustion system).

This configuration, along with the use of inverters on the water pumps and air fans, manages the supply flow rate from the economiser to the pre-heater based on the incoming temperature of the supply water, the external air temperature, and the load. All of this makes for consistent performance regardless of external conditions, always ensuring a constant outgoing flue gas temperature of 80°C.

The first part of the analysis will assess the relationship between the supply hot water temperature and the flue gas leaving the boiler.

The essential feature of HE Smart is its capacity to control the outgoing flue gas temperature. In fact, the HE Smart was designed to manage a settable Tflue gas, usually of 80°C, for all operating conditions and at any boiler supply water temperature with a tolerance of ±15°C compared to the design temperature.

Two essential aspects define this configuration:

1. Low temperature of the flue gas leaving the boiler, implying extremely high heat recovery and, as such, high generator efficiency;
2. At 58°C when there is methane gas combustion, the flue gas leaving the boiler forms acidic condensate. The 80°C temperature ensures a ΔT greater by at least 20°C compared to the condensate condition, preventing the flue gases from forming acidic condensate along the entire length of the flue, which is often uninsulated.

Instead, the solution that only has the economiser is extremely sensitive to variations in the supply water design temperature. As shown in the following images (Figures 1- 2), even a minor variation in the water temperature causes a significant variation in T flue gas and, consequently, in performance, leading to reductions of 1÷1.5 η percentage points.

Figure. 1 – Flue gas temperature vs Supply water temperature

We can state, therefore, that “ECO” heat generator behaviour is dictated by the thermodynamics of the economiser, which, being a static exchanger, is unable to actively modulate the amount of absorbable heat, thereby remaining inversely proportional to the incoming water temperature.

In the image in Figure 2, HE Smart generator behaviour is summarised in one point, as the outgoing flue gas temperature is defined and can be configured, usually set to 80°C with a corresponding theoretical performance of 97%.

Figure.2 – Variation in steam generator efficiency based on TFlue gas

Figure 3 – Variation in generator efficiency based on T boiler supply water

The image in Figure 3 clearly compares the two previously described technologies when the boiler supply water temperature varies, emphasising behaviour at low and high temperatures.

As we can see, low supply temperatures facilitate heat recovery from the flue gas; however, this creates potential issues with acidic condensate, as described previously. This aspect is often disregarded but, at the same time, it must be assessed when choosing a steam generator.

Condensate can cause harsh corrosion that attacks the components that are directly in contact with it, causing production downtime and additional expenses for maintenance or, in the most critical cases, replacement.

It is very important to emphasise that the performance gap increases as the supply temperature overheats, until reaching more than 4 percentage point for temperatures exceeding 140°C. This operating condition is typical of many industrial processes, like paper factories, and is extremely critical for intense, efficient heat recovery from hot flue gases.

Figure 4 shows the behaviour of the two high-efficiency solutions and the standard solution when the load required by the utility changes to a supply water temperature of 140°C.

The image shows how the HE Smart curve reaches values close to maximum efficiency when the generator is at 40% of its maximum load, highlighting a difference of 3 percent points for all the load values.

As previously described, all of this is possible thanks to the advanced retroactive control of the heat recovery distributed among the various heat recovery sections that, along with the inverters on the pumps and fan, allow for partial load that is always optimised.

Varying efficiency based on the load is a crucial aspect in terms of the effect on annual energy savings, as it is this factor and not the peak value in certain conditions that is responsible for savings on annual fuel consumption.
Until now, this aspect has always been ignored due to the lack of proper instruments on systems and steam generators.

Today, the multitude of incentives for energy efficiency and Industry 4.0 systems has encouraged those supplying these technologies to concentrate on developing reliable, efficient, and -above all- affordable solutions

Figure 4 – Variation in generator efficiency based on the load required by the utility (steam)

Figure 5 – Annual methane consumption based on T supply water: comparing the technologies

Figure 5 shows annual methane consumption as the boiler supply water temperature varies.

As we can see, HE Smart has the lowest consumption in the entire temperature range, highlighting an increase in this trend as the water temperature increases.

This trend is made even more evident by the images in Figures 6 and 7, which show how much natural gas is saved by using the two proposed solutions, as well as a comparison between the two.
As shown in Figure 7, compared to the ECO solution, the HE Smart solution can lead to savings from 10 to 35 thousand euro a year, with supply water ranging from 80°C to 150°C.

Over the course of the presentation, the importance of the steam generator being highly energy efficient has been emphasised several times; Table 2 and the image in Figure 8 deal with this aspect in financial terms, that is, the savings that can be made from every single percentage point of increased efficiency.
It is important to highlight that these values, while high, are absolutely conservative, as they speculate average machine performance equal to the peak value, which is easily achievable with the HE Smart but impossible with the other solutions.

Figure 8 – Annual methane savings per percentage point of increased efficiency

It is clear how every percentage point has a different financial value depending on the boiler supply water temperature as, by increasing the temperature, the amount of heat needed to bring the boiler to supply conditions decreases, along with methane consumption. At the same time, it becomes more difficult to recover heat from the outgoing flue gas and this is where the adaptive HE Smart solution provides the best performance, with a 4.5 percentage point difference in efficiency compared to the ECO solution and a subsequent annual savings of about €35,000.

In any case, it is important to emphasise the importance of increasing generator efficiency even by a single percentage point; in fact, what might seem like a tiny advantage in terms of performance generates an average annual savings of about €8,000.

Further HE Smart technical developments/advantages

Specific system to optimise transients

This device, controlled by the Optispark system, becomes essential when the business frequently shuts off the generator (night/day – weekends), causing the generator and the steam line to depressurise.

In these situations, the pressure and, therefore, the temperature of the steam inside the generator drops abruptly and must be restored the next time the machine is started up.

This situation has various consequences:

1. The generator needs to be started up much sooner than when it will actually be used to allow the water inside of it to reach set point pressure conditions, especially for machines containing large quantities of water;
2. A qualified operator must be there to supervise every time the generator is started up, with increased financial and organisational implications;
3. Starting up the generator in atmospheric pressure conditions that are much lower than set point conditions consumes considerably more fuel.

Controlled by the Optispark system, this solution uses various software and hardware to reduce heat dispersion from the generator to the flue and/or steam line, preventing it from depressurising and considerably limiting heat/pressure loss.

Figure 9 – HE transient management system

As we can see from the image in Figure 9, when there is no transient control system, the steam pressure at start-up after being shut down is quite far from the operating set-point, most often resulting in temperature and pressure being close to ambient conditions.
Using this system provides significant savings on methane gas consumption, as pressure drop is contained at a mere 3 bar.

Furthermore, by preventing continuous temperature highs and lows, there is no thermal stress to the pressurised structure and much less time is required to bring the system back to operating conditions.
To this end, the generator can be started up in two ways: “fast” if set point conditions need to be reached quickly or “economy” for financial savings.

In addition, in systems without an operator, setting a night-time set point at lower pressure than when operating, though not by much, the system is managed in “stand-by” conditions, as it can re-start safely without an operator.

The transient control system has often proven an efficient solution to optimise system operation, contributing to significantly reducing the fuel consumption required for frequent load changes.