Answers To Common
Questions About Steamer
"What is a reasonable life for a kitchen steamer boiler?"
Service life depends on more factors than you know about and have absolutely NO control over. In a commercial or industrial boiler, the ideal feedwater will have no minerals and no dissolved oxygen. Because steam from the kitchen steamer boiler comes into direct contact with food, the water that goes into the boiler must be potable and is usually taken from the kitchen tap water line. Water that is safe to drink is not necessarily good for a boiler. Two things in boiler feedwater determine the life of a steamer boiler - dissolved minerals and dissolved oxygen.
"What is so bad about dissolved minerals?"
In a kitchen steamer boiler, tap water goes in but only steam comes out. All the minerals in the feedwater stay in the boiler. IF the boiler is operating in a bad water district where the mineral content of the water is 400 part per million, making 300 pounds of steam per hour, for each hour 1/8th of a pound of minerals/mud stays in the boiler. For an 8 hour day of operating, 1 pound of mud stays in the boiler. Even though most of the mud will be blown out when the boiler blowoff dumps all the water when the unit is turned off, a still significant amount will get baked on the tubes and lower tube sheet during the day's operation. Using relatively good water with only 40 parts per million hardness, a pound of mud would be collected after 80 hours of operation (roughly every two weeks). For a kitchen steamer boiler that sees hard use, 14 hours a day, seven days a week, you can see that it doesn't take very long to scale up even with relatively good water. Mineral scale is a very good insulator and a 1/8th inch layer can reduce the heat transfer by 50%. If the heat doesn't transfer to the boiler water, it goes up the stack as hot gas or heats the boiler tubes to high temperatures that cause the steel to start falling apart metallurgically or get "mud burned".
"What is so bad about dissolved oxygen?"
Oxygen loves to eat iron. Since steel is mostly iron, if you have a steel boiler with oxygen in it, rust will form. Oxygen needs water to easily react with iron. They don't paint steel bridges in Arizona because it doesn't rain and there is no water in the air. Water in a boiler is the perfect environment for oxygen to form rust. Not only is there water, but high temperature and high stress as well to accelerate the reaction.
"Why can't chemicals be used to protect the kitchen steamer boiler?"
Chemicals can be used. The
problem is the cost. Because the steam for the boiler has direct
contact with food, food grade boiler chemicals have to be used. Food
grade boiler chemicals cost 5 to 10 times more than the same chemicals
used in a laundry boiler. Plus the daily attention required to provide
correct chemical dosing, the total cost for the chemical treatment far
exceeds the cost of replacement of the rusted out boiler.
"Why can't the boiler be made out of stainless steel so they won't rust so quickly?"
The simple reason is because the
ASME Boiler Code doesn't allow it. But there is a good reason.
The main element that makes a steel formula "rust resistant" is chromium.
The water source is regular tap water which invariably is chlorinated.
Steel in a boiler by definition has fire on one side, water on the other,
and a vast temperature difference between. This causes one side of
the steel to try to expand more on the fire side than on the water side.
This sets up internal stresses in the steel plate.
If there is chromium in the steel plate, chlorine in the water and high stress and high temperature, the chrome and chlorine atoms like to marry each other forming a chromium chloride salt. This chromium chloride salt tends to form in the grain boundaries of the steel at a rather rapid rate. Cracks will form in the grain boundaries, filled with these salts all the way through the plate. After only several months of operation, the cracks will be long enough and wide enough to allow water to leak through causing the boiler to fail.
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