Most boiler problems we are called in to fix are sizing problems, not equipment problems. The unit works exactly as designed. It was simply asked to do the wrong job.
This guide sets out how industrial boiler sizing is actually done, what information the calculation needs, and the three mistakes that account for most of the badly specified plant in Saudi facilities. It is written for the person preparing a specification rather than for a purchasing department comparing quotes.
This guide assumes the type of boiler is already settled. If it is not, start with choosing between steam, hot water and thermal oil, then come back here for the capacity calculation.
Start from the process, not the building
A boiler is sized from what it feeds, never from the size of the building or the plant room it will sit in. That sounds obvious and it is routinely ignored, usually because the building is the only information available early in a project.
Work out the connected load first. For steam, list every item of equipment the boiler serves and its steam consumption at full duty: sterilisers, jacketed vessels, dryers, presses, heat exchangers, humidification, washdown. For hot water, list the heating circuits and process demands with their flow rates and temperature rise.
The sum of that list is the connected load. It is not the boiler size. Almost nothing runs every item at full duty at the same moment, and sizing to the connected load is the single most common route to an oversized boiler.
Diversity: what actually runs at once
Diversity is the difference between everything that could run and what genuinely runs together. Getting it right requires knowing how the plant is operated, which is why sizing conversations should include the people who run the shift, not only the people specifying the equipment.
Ask three questions. Which items run continuously, and which are intermittent? What is the worst realistic combination, on the busiest shift of the busiest day? And are there start-up loads that occur once a day but are much larger than anything in normal running?
That last one catches people out. Bringing a cold system up to temperature in the morning can demand far more than steady-state running ever does. If the plant has to be productive within a fixed time of switch-on, the start-up load may set the boiler size rather than the process load.
Peak, average, and why both matter
The peak sets whether the boiler can meet demand. The average sets what it costs to run and how long it lasts.
A boiler sized generously against peak spends most of its life at part load, cycling on and off. Short cycling wastes fuel on every purge and light-off, stresses the burner, and shortens the life of the unit. A correctly sized boiler running near its design load is more efficient than a larger one running at part load, even when the larger one carries the better efficiency figure on paper.
Where peak and average are far apart, the answer is usually not one large boiler. Two smaller units in sequence, or one boiler plus adequate storage, will hold the peak while running efficiently the rest of the time. Modular arrangements also mean a fault takes out part of the capacity rather than all of it.
Pressure and temperature: specify what the process needs
Working pressure is set by the most demanding item on the system, plus the losses in getting steam to it. Distribution pipework, control valves and elevation all take their share, so generating at exactly the pressure the process requires will leave you short at the point of use.
The instinct is then to specify a much higher pressure for safety. That has costs: higher generating pressure means higher flue temperature, greater standing losses and more heat lost through the distribution system, every hour the plant runs.
Specify the pressure the process needs, add a realistic allowance for distribution losses, and no more. If one item needs far more pressure than everything else, look at whether it can be served locally rather than raising the pressure of the entire system.
Fuel: confirm supply before you specify
Gas availability is not uniform across Saudi industrial areas, and it is worth confirming at the site rather than assuming from the location.
A plant specified for natural gas on a site with no connection ends up on a temporary fuel arrangement that costs more per unit of heat for the whole life of the installation. That is an expensive way to discover a supply problem.
Where supply is uncertain, or the plant cannot afford to stop if supply is interrupted, dual-fuel capability is usually worth its additional cost. Where combustion on site is impractical, because of a flue, fuel storage or permitting, electric thermal oil reaches process temperature without any of it.
Feedwater and blowdown: the part that gets left out
Feedwater treatment is the most commonly omitted item in a boiler specification and the most reliable cause of a boiler failing to reach its rated life.
Scale on heat transfer surfaces forces the burner to work harder for the same output, so efficiency falls steadily and invisibly. Dissolved oxygen corrodes the pressure vessel from the inside. Neither shows up immediately, which is precisely why both get deferred.
Blowdown also has to be in the sizing calculation. Removing concentrated water from the boiler removes heat with it, and the worse the feedwater quality, the more blowdown is required. On poor feedwater this is a real percentage of output, not a rounding error.
Get the feedwater analysed before the boiler is specified, not after it is installed.
The three mistakes that cause most of the trouble
Oversizing. Adding a safety margin to a figure that already contains one. Connected load, then a diversity allowance nobody trusts, then a contingency, then rounding up to the next model. Four cautious decisions produce a boiler twice the size it needed to be, and it will cycle for the next twenty years.
Sizing from the building. Floor area and roof height tell you nothing about steam demand. They get used when nobody has collected the process figures, and the result is a guess wearing the appearance of a calculation.
Treating water treatment as optional. It is the cheapest item on the specification and the one that determines whether the most expensive item reaches its rated life.
What to send when you ask for a specification
- The process the boiler serves, and whether the load is steady or intermittent
- Connected load, item by item, with duty figures
- Peak and average demand, and when the peaks occur
- Start-up requirement, and how quickly the plant must be productive from cold
- Required working pressure and temperature at the point of use
- Fuel actually available at the site: natural gas, diesel, LPG, or electric only
- Feedwater analysis, and any existing treatment
- Plant room dimensions, including access for installation and future removal
- Whether this is a replacement or a new installation
Incomplete information is normal at the first enquiry. We would rather start the conversation and fill the gaps than quote against assumptions.
Send your figures and we will specify against them
Hi-Best has supplied and installed industrial boilers across Saudi Arabia since 1995. Send what you have and we will come back with a specification rather than a price range.
Call +966 54 433 2843 or request a quotation. You can also see the six Ekotek series we supply.