Hot stuff, hot stuff... Can't get enough
Hot stuff, hot stuff... Can't get enough.

 

("Hot Stuff," by the Rolling Stones, 1975)

 

UNDER-APPRECIATED HOT WATER

In the vast majority of the United States, the utility of "hot water" is taken for granted.

Hardly anybody has a memory of hauling water from a well and then heating it up to wash dishes, hand-wash clothing, or fill up a stand-alone bath tub. Nowadays the utility of hot water is only missed when a natural catastrophe disrupts its on-demand delivery for more than a day.

After a day without hot water, everybody is grumpy and  'wants more of the hot stuff.'

So show some respect for the electric water heater that is located in that dark closet or garage of your house. If you haven't taken a look at it lately, it looks a bit like the picture at the top right.

If your house water heater doesn't look like that there's a good reason:

Many homes in the United States have access to natural gas as a utility source. Natural gas can be combusted (burned) in a natural gas boiler to indirectly heat water.

A picture of a natural gas-fired boiler is in the photo above; the boiler is the gray, rectangular box.

 

INDUSTRIAL USES OF HOT WATER 

The Control Room Operator in the picture to the left  is responsible for the boilers on a college campus. The boilers that this Control Room Operator is accountable for are much bigger than the common house boiler.

When he does his job correctly, hot water is distributed on demand to the areas of a college campus that need it... like the cafeterias, bathrooms, dorms and laboratories.

 

MANY INDUSTRIAL USES OF STEAM

In process industry, boilers are used to raise the temperature of water until it changes phase and becomes steam, the vapor that hot water turns into.

Some of the uses of steam are enumerated below.

1. Steam can be used to heat  process streams indirectly (without mixing directly with the process stream).

The picture above shows a kettle-type reboiler.

PTOA Readers and Students can follow the arrows on the schematic to learn that this reboiler uses steam to heat up the product that comes out of the bottom of a tower.

This product is logically labelled "Bottoms." PTOA Readers and Students can see that the Bottoms Product flows from the tower into the bottom right side of the reboiler. 

The Bottoms Product collects in the reboiler to a level that covers up the tubes of the heating element. Excess Bottoms Product flows out of the reboiler on the bottom left side.

The reheated process stream vapor returns to the tower.

The steam enters the reboiler, flows through the hair-pin tubes of the reboiler's  heating element...never mixing directly with the process stream...and then exits at the bottom of the reboiler.

PTOA Readers and Students will learn more about reboilers in future PTOA segments that cover distillation. For now just remember that steam can be used in reboilers to indirectly heat up process streams.

 

2. Steam can be used to strip impurities out of gases and liquids in towers that are logically called "strippers."

The stripper in the photo on the left uses steam (the red arrow)  to strip VOCs (Volatile Organic Compounds, the pink arrow) out of the feedstock to the tower(lime green arrow).

• Since the product (blue arrow) coming out of the bottom of the tower is labelled "Clean Wastewater," smart PTOA Readers and Students can deduce that the feedstock was composed of wastewater plus impurities called VOCs. The steam was used to strip the VOCs from the feedstock.

Future PTOA segments on separating systems will cover steam stripping in more detail.

 

3. Steam and methane (aka: natural gas) can also be converted into hydrogen.

No kidding! Steam (H2O) and methane/natural gas (CH4) can be chemically broken up and reassembled to make carbon monoxide (CO)  and hydrogen (H2) by the below reaction:

Future PTOA segments covering natural gas conversion reactions will review "Stream Methane Reforming" (the nick name of the reaction above) in more detail.

Hydrogen generating plants are found in many processing complexes.

Hydrogen also has a future as a motor fuel. PTOA Readers and Students may be driving hydrogen-fueled vehicles in their lifetime.

 

4. As shown in the below schematic, high pressure steam can be allowed to expand...increase its volume...in a piece of rotating equipment called a steam turbine.

Steam turbines can be coupled to the shaft of a compressor, pump, or electricity generator.

When the steam turbine spins, so does whatever is connected to it.

The steam turbine can be controlled to spin faster or slower.

Since they are connected, whatever the steam turbine is coupled with can be made to turn faster or slower as well.

In other words, a steam turbine can become a "variable-speed driver."

PTOA Readers and Students will learn more about drivers in future PTOA rotating equipment segments.

In the below schematic, the boiler is the light purple box.

The green part in the below schematic shows the product from the boiler, high pressure steam, flowing into the green steam turbine.

In the above graphic, the steam turbine is coupled to an electricity generator which is outlined in blue.

This steam turbine is being used as a variable-speed driver to spin the electricity generator.

The steam exits the steam turbine as much lower pressure steam and "condensate." Condensate is steam collapsed back into its water form.

The temperature of the exiting steam from the steam turbine is cooled until all of it changes state back into condensate. Then the condensate is recycled back to the boiler to be transformed into steam again and again.

 

5. Steam can also be used as 'heat tracing.'

Heat tracing is attached to the outside of pipes to make certain that the process fluid inside the pipe does not freeze when the outside ambient temperature drops into the frigid zone.

 

6.Steam is also used to sanitize equipment and make certain that a non-reactive, inert atmosphere is present as a safety precaution.

 

Take Home Messages: The purpose of boilers in the process industries is to make high pressure steam from water.

Changing the state of water and producing steam is accomplished by raising the temperature of the water in the boiler.

The many industrial uses of steam explain why boilers are found in processing facilities. Some of the industrial uses of steam are:

• Indirectly heating up process streams (for example in a reboiler).
• Stripping the impurities out of feed streams in equipment called strippers.
• Reacting with methane (CH4) to make hydrogen gas (could be fuel for H2 cars!).
• Being expanded in a steam turbine which is working as a variable speed driver.
• Keeping pipes from freezing when used as steam tracing.
• Insuring a non-reactive atmosphere exists and sanitizing equipment.

 

©2015 PTOA Segment 00023
Process Industry Temperature Changing Equipment

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