Steam

Steam

 

Like all other substances, water can exist in several states— solid, liquid, or vapor— depending on its temperature and pressure.  Steam is the name for water in its vapor state.  Steam is created by heating liquid water to a point where the intermolecular forces between the molecules are broken, allowing the molecules to move apart.  The temperature at which the first water molecule transforms to vapor is called the boiling point.  As more heat is added to the liquid/vapor mixture, the temperature remains at the boiling point until all of the liquid is transformed to saturated steam.

Steam Benefits

Steam is the most commonly used heating medium in thermal maintenance applications because of three key benefits:

  • Significant energy storage.  A significant amount of heat energy is required to transform liquid water to steam.  This energy is stored in the steam.  When saturated steam comes into contact with a cooler object, heat is transferred from the steam, and very large heat transfer rates are produced as the steam condenses into liquid water (often referred to as condensate).  Compared to hot oil— the second most commonly used heating medium— condensing steam produces 10X as much energy per pound.
     
  • Constant temperature.  The steam/condensate temperature remains at the boiling point until all of the steam is transformed back into liquid water.  So, for a given steam pressure, the heating medium remains at a constant temperature.  This preserves a maximum temperature difference between the heating medium and process, which maximizes the heat transfer rate.
     
  • Process by-product.  Often, steam is created as a by-product of a heat exchange operation designed to cool a process stream.  This is the case with a waste heat boiler.  Since this steam is already available, it is usually more economical to use it rather than another as a heating medium/technology.

 

Major Components of a Steam Jacketing System

Every steam jacketing system consists of the following major components (at a minimum):

Boiler
In the boiler, heat is added to liquid water to generate steam at a certain pressure.  Boiler pressure is used to control the thermal capacity of the steam and to motivate the steam through the jacketing system.

Steam Header
Steam exits the boiler through piping referred to as the steam header.  The header also serves as a reservoir that feeds steam to the individual heating circuits.  The steam header must be large enough to virtually eliminate pressure drop between the boiler and the beginning of the circuit.

Steam Manifold
Smaller diameter piping connects the steam header to a steam manifold.  The manifold serves as the branch point for supplying the individual heating circuits.  Steam manifolds commonly have 4-16 branches, and each branch contains an isolation valve.  A steam trap is located at the bottom of the manifold for removing any condensate from the manifold.

Heating Circuits
The heating circuit is comprised of a group of heating elements (jacketed pipe, bolt-on jacketing, or tube tracing) which are connected in series.  Typically, pre-insulated tubing is used to transport steam from the steam manifold to the first heating element in the circuit.  Flexible metal hoses are commonly used to allow the steam to flow from one heating element to the next element in the circuit.  Pre-insulated tubing is also used to transport steam and condensate from the last element in the circuit to the condensate manifold.  The length and configuration of each heating circuit must be carefully designed and analyzed to ensure fresh steam is supplied to the jacketing system before the steam has lost too much pressure.  (See Maintaining Steam Pressure)

Condensate Manifold
The condensate manifold resembles the steam manifold with the exception that each branch contains a steam trap in between 2 isolation valves.  The steam trap serves to remove condensate from the system while “trapping” steam in the system.  The steam trap has critical role in determining the overall success of the steam jacketing system.  (See Steam Traps) 

Condensate Header
Condensate from each of the condensate manifolds flows into a common pipe header referred to as the condensate header.  The condensate header returns liquid water to the boiler so that it can be reheated into steam.  Special attention must be given to the piping design of the condensate header to avoid excessive pressure drop which could impede operation of the steam jacketing system.  This can have particular impact upon the type of steam trap that can be used with the system.  (See Steam Traps)

Steam

Key Considerations

There are 3 key considerations that must be analyzed in a steam jacketed system:

To visualize the flow of steam/condensate through a steam jacketed system, please click on the following link: