Steam is made by adding enough heat to water to change it from a liquid into a gas. This is usually done by using a boiler. Boilers come in several types and configurations. The most common are fire-tube and water-tube boilers. Either style uses piping (tubes) in which the heat source is on one side of the tube and the water being heated is on the other side of the tube. The fire-tube boiler is most commonly used and has the fire side inside the tubes and the water on the outside of the tubes. This boiler may be designed so that the fire passes through the length of the boiler from 1 pass to as many as 4 passes. The transfer of heat from the fire (dry) side of the tubes to the water (wet) side of the tubes should not be restricted by buildups of soot or corrosion that would reduce the heat transfer rate.
Fuel burned by the boiler is usually some type of gas or oil. The most commonly used fuel is natural gas, but propane and different types of oils also may be used. In each case, the fuel supply system feeding the boiler burner must be designed for the type of fuel used. Some boilers may be set up to burn more than one fuel if a single fuel supply isn’t available by installing a parallel fuel system to handle a second fuel. It is important that the fuel burns efficiently by making sure all the fuel burns in the process and that the tubes are clean with no buildup on either the fire or water surfaces of the tubes. The photo at the top of this page shows a cut-away view of a fire-tube boiler and its accessories.
Incomplete combustion of the fuel may cause soot to buildup on the fire side of the boiler tubes and reduce the heat transfer from the fire side to the water side. It also means that not all the fuel is burned and some escapes through the boiler exhaust stack as flue gas. This reduces the boiler efficiency and requires additional fuel to make the desired amount of steam. If incomplete combustion occurs, excess oxygen is in the flue gas.
When burning natural gas, sufficient air must be present for efficient combustion. But if the escaping exhaust gasses contain excess oxygen, some of the gas is not being burned and escapes with the flue gasses. If the flue gasses are at a temperature of 350 degrees F and as little as 1% excess oxygen is present, up to 17% of the natural gas is not burned in the boiler burner and escapes with the flue gasses. See the table on page 51 for potential not combusted fuel if excess oxygen is found in the flue gasses.
Treatment of Makeup Water
Proper treatment of makeup water to the boiler and steam system is also important as it is necessary to prevent corrosion, scale and other deposits in the boiler and steam system. This treatment is part of the process to keep the steam pure and the boiler and system free of corrosion or buildups of foreign materials. A boiler and the steam system have three potentially destructive elements: oxygen, carbon dioxide and solids.
The first move is to eliminate as many problems with the water before it enters the boiler. The most common pre-treatment is using a water softener to remove grains of hardness and produce a soft water. Prior to the boiler there is a feed water tank into which returned condensate from the system is mixed with new (make-up) water. By injecting this tank with steam it is possible to de-aerate the water and remove excessive oxygen in the feed water before it goes to the boiler.
As the water is heated and turned into steam, free oxygen is released into the steam. The heat also causes carbon dioxide to be released from the solids and the solids settle to the bottom of the boiler. Other impurities, with proper chemical treatment, float on the surface of the water at the top in the area where it turns to steam. A skimmer trough is installed at the water level in the boiler and the impurities are collected and discharged from the boiler. This is called continuous blowdown. A second type of blowdown is done when valves on the bottom of the boiler are opened and quickly closed two to three times every 24 hours. This flushes the solids that have settled out of the bottom of the boiler.
Properly treated water in the boiler should have tests in the following range:
- Dissolved oxygen – less than 7 ppm (parts per million)
- PH level – 7-10
- Total hardness – 0 ppm
- Total alkalinity – 400 ppm max.
- Dissolved solids – 3,500 ppm max
- Silica – 150 ppm
- Oily matter – 1 ppm
- Total iron – .05 ppm max.
It is important that the boiler water be checked daily and proper chemicals added to achieve these levels. A scale buildup on the water side of the boiler tubes decreases boiler efficiencies by requiring more fuel to produce the steam: 1/16 inch of scale requires 15% more fuel, 1/8 inch of scale requires 20% more fuel and 1/4 inch of scale requires 39% more fuel. Improperly treated water also may lead to deterioration of piping, controls, and traps throughout the system.
The boiler needs to have a low fire or “non-using” level so that it doesn’t operate at high fire and then shut down completely when no steam is required. Operate the boiler at the least pressure needed to deliver steam to the system. Avoid short cycling times when the firing rate goes from high to low and back again. To avoid these problems, the boiler should be equipped with modulating controls to adjust the firing rate based on demand.
A term that is used in steam is “steam quality.” This term is used to describe the amount of vapor and/or water in the steam. A 100% steam quality means that the source is all steam and no water. A 90% quality steam would have 90% steam and 10% water. This term becomes important when you look at the quality of the steam, where it is used and whether that quality makes a difference. The inclusion of water in the steam at the use point happens due to improper trapping of the steam lines or heat loss through under insulated piping.
To view an enlarged version the chart, click here.
Keys to High Efficiency
Following are some points to keep in mind to operate the steam system and get the highest efficiency and control costs.
1. The continuous blowdown water from the boiler is hot and cannot be sent to a regular sewer system unless it is cooled. A heat exchanger may be installed on the continuous blowdown stream to preheat the water going to the boiler while cooling the blowdown.
2. An economizer (heat exchanger) may be installed on the flue gas exhaust of the boiler to preheat the makeup water while reducing the flue gas temperature. The final temperature of the flue gas should not be less than 275 degrees F.
3. Maintain condensate traps in good working order. A leaking trap may allow steam to blow through directly into the condensate return system. This steam is then lost to the system and requires that additional steam be made to replace that lost through the leaking trap. A leaking trap with a ¼-inch diameter opening in a 100 psig system will leak 163 pounds per hour. In a year’s time of constant leak on a system operating 24 hours per day, 1,427,880 pounds of steam may be lost from this single defective trap.
4. Keep all steam piping properly insulated to make sure steam reaches its destination in the best quality. Assume you have a 100-foot section of 6-inch steam pipe with a system pressure of 100 psig. The line temperature is 300 degrees F and the boiler operates 6,000 hours per year. If the pipe is uninsulated, the bare pipe will lose 508mm BTUs per year, but with 1 inch of good insulation, that loss would be reduced to 46mm BTUs per year. Thicker insulation will further reduce the loss.
To summarize, the selection of a properly equipped boiler with the addition of a water softening system and proper chemical treatment is a must. Pre-heat the boiler makeup water using an economizer on the exhaust stack. Installing a heat exchanger on the continuous blowdown also may help preheat the boiler makeup water. Establish a monitoring program to make sure all traps and other accessories are operating properly. Properly insulate and maintain steam supply lines. A properly designed and operated steam system will keep energy costs as low as possible.
Fred Fairchild is feed science professor emeritus in the Department of Grain Science at Kansas State University. Prior to coming to Kansas State in 1994, he worked in the industry designing, constructing and commissioning numerous mill facilities. He is a licensed professional engineer. He may be reached by e-mail at firstname.lastname@example.org.