An Engineering Extension energy feature
An Engineering Extension energy feature
By Tom Logan
You're setting up a new business venture in a classic 1920s structure that uses steam for heating.
Your first thoughts are of high energy costs, or even a replacement -- with a system you are more comfortable with. Don't be alarmed; the advantages of steam heating are as great as ever.
Steam is an efficient heating medium. It can transfer large amounts of heat from a boiler to the space you are heating. Once the water is at boiling temperature, the same amount of energy required to convert the boiling water to steam is released as heat when the steam condenses back to water.
Let's look at the basic components of your steam heating system so we can understand their function.
The easiest way to explain how a steam system works is to follow the path of the steam through the system. The major components include boiler, supply piping, radiators or heating coils, steam traps, return piping, and condensate sets.
The boiler is the heart of the system. A boiler heats water until it vaporizes to steam. Since it operates in a closed-loop system, the boiler is the start and finish point for the steam.
Controls are critical to the safe operation of the boiler. Boiler controls can be divided into three categories: operating, limit and safety.
Operating controls regulate the normal functions of a boiler. This includes the operation of the gas valve, burner and combustion air dampers, and shutting down the system when demand (desired room temperature, for example) has been met.
Limit controls stop the boiler when any one of the following occurs: Steam pressure reaches its set point, amount of water in the system drops below a safe level, stack temperature is too high, fuel (gas or fuel oil) pressure is too high or low.
Safety controls, the last type found on boilers, stop fuel flow in case of ignition failure, main flame interruption, mechanical draft or electrical circuit failure.
All boilers are equipped with two safety devices -- low water fuel cutoff and boiler safety valve.
The low water fuel cutoff stops the burner if the water level drops below its required operating level.
When a boiler is fired under low water conditions, the water jacket (divider between firing chamber and water) can burn through. Water floods the firing chamber, leading to the rapid formation of steam. A steam explosion may result because steam expands to approximately 1,600 times the volume of water.
Safety valves are critical. They open automatically when boiler pressure exceeds the maximum allowable working pressure (MAWP). Fifteen pounds per square inch (psi) is the safety valve setting used on most low pressure boilers.
From the boiler, steam travels through the supply piping to the radiators.
There is usually a strainer and a control valve at the equipment. The strainer removes dirt and scale picked up from the piping. Dirt and scale can plug or damage the control valve and steam trap.
The control valve can be manual or automatic. With a manual valve, you open or close it as the need for heat increases or decreases. A thermostat operates automatic control valves. Automatic control is more sensitive to temperatures and more convenient for the user.
When the steam reaches the radiator or coil, it condenses and heat is released. Condensate must be removed quickly from the heating equipment. This is where the steam trap does its job.
A steam trap holds steam in the equipment until it condenses. After it does, the trap assures rapid flow of the water to the return system.
The trap also is important in the system operation because it allows the non-condensable gasses to pass through the system and be vented from it. Noncondensable gasses are composed of oxygen and carbon dioxide. Both are leading causes of system deterioration.
After the condensed water has passed through the steam trap, it flows back to the boiler in the return system. The return piping should slope to the boiler. Return piping should be kept in top condition and should be insulated to reduce heat loss. Every degree of heat lost must be added back before the water will change to steam again.
On a gravity return system, gravity returns the water to the boiler. This type of return system operates at only one to two psi pressure.
Higher pressure and newer steam systems use a pump type return system. Condensate from the radiator flows by gravity to a condensate receiver, and then is pumped to the boiler.
Condensate sets normally consist of a storage tank (receiver) and pump. A switch in the low water cutoff signals the pump when the boiler needs water.
Usually there is a float in the condensate set to control make-up water level in the receiver. If there is inadequate condensate coming back from the system, the make-up valve opens and brings the water up to the required level.
If you operate and maintain a steam system properly, it can last for years. Systems that have been in service for 50-75 years are still common in Kansas communities.
If your business is in an older structure that uses steam heating, there is no reason to not expect the system to continue as an excellent heating source.
Like all mechanical operations, steam systems require regular maintenance and repair. Each component serves a specific function, and if the components don't operate properly, efficiency and performance of the entire system suffers.
An example of a common repair for steam systems is fixing leaks. You must repair leaks in the supply and return piping to prevent heat and water losses.
Besides checking for leaks, you should watch for deposits of dirt and scale.
If the surfaces of the heat exchanger are covered with dirt and scale, they transfer less heat. As little as 1/64-inch of scale can lead to a 1 percent loss in efficiency. Soot build-up can foul the combustion side of the boiler, also.
You can remove scale by either brushing or using chemicals, or both, depending on the extent of scaling and its location in the boiler. You can usually remove soot by brushing. Commonly, boilers are either cleaned before the heating season or at its end.
The quality of water used in boilers is of prime importance. Hard water leaves scale, so you must either soften or treat it.
Additionally, the water may need chemical treatment to remove oxygen and carbon dioxide which can cause serious damage to the system. Oxygen promotes corrosion, and carbon dioxide in water forms carbonic acid that corrodes piping. Venting the system at the condensate set will also reduce gases in the system.
The chemical composition of the boiler water will cause serious health problems if the water supply is contaminated. Install a backflow preventer in the domestic water line feeding any boiler room equipment. This prevents contaminated water from siphoning back into your primary water supply.
Check the boiler burner to assure that flue gas temperatures and excess air are at appropriate levels. You can do this yourself if suitable equipment is available, or you can contact a reputable service contractor.
Kansas boiler code requires two low water cutoffs on all new installations and major rebuilds. One control is set slightly lower than the other. The upper control may reset automatically, but the lower must be manually reset.
You should test the low water fuel cutoff and safety valve regularly -- weekly for the low water cutoff, monthly for the safety valve.
To test the low water cutoff, the burner must be operating. Open the low water cutoff flush valve fully. When the water level drops below the set level, the cutoff should shut down the burner. If the boiler continues to fire, repair or replace the cutoff immediately.
To test the safety valve, you must manually open the valve's try lever, which is located on the top of the valve. Let the steam blow for 5-10 seconds, and allow the valve to snap shut. If it does not seal tightly, replace it.
Once a year, test the safety valve as follows: Shut down the boiler and let it cool. Completely fill the boiler with water and then increase pressure slowly until the safety valve lifts. The lift pressure should not exceed the maximum allowable working pressure (MAWP).
If the safety valve operates correctly, restore the water level to normal so it can be put back in service. If you need a replacement, use only a new or factory rebuilt, certified valve. Kansas boiler code does not permit field rebuilding of safety valves by individuals.
Insulate steam supply and condensate return lines. This will slow heat loss and formation of condensate in the supply piping. It also assures delivery of steam to the desired location, and lightens boiler loads by reducing the amount of heat required to raise the condensate to steam.