Heat Transfer

Heat Transfer

 

Temperature and Heat Transfer
Temperature is a measure of the molecular activity of an object.  The hotter an object is, the more its molecules will vibrate and the higher its temperature will be.  If two objects are at different temperatures, the hotter object will transfer thermal energy to the cooler object until they reach the same temperature.  This exchange of thermal energy is known as heat transfer, and a temperature difference is the driving force for heat transfer.

Heat Transfer Rate
The rate at which heat is transferred between two objects at different temperatures can be determined from the following simple equation:

Where,

q is the amount of heat transferred between the two objects
U is the ease with which heat is transferred
A is the surface area in contact between the two objects
ΔT is the temperature difference between the two objects

 

 

 

U is also referred to as the Universal Heat Transfer Coefficient and can be considered to be the inverse of the thermal resistance to heat transfer.  A system which transfers heat easily (low thermal resistance) would have a high U.  A system which impedes heat transfer (such as a very thick layer of insulation) would have a low U.  The U value must be determined for the entire system using industry standard, empirically based heat transfer formulas.

Heat Transfer in a Piping System
A process fluid flowing through a piping system represents a common heat transfer problem.  If the temperature of the pipe is greater than the temperature of the surrounding ambient air, heat will transfer from the pipe to the ambient, thus reducing the fluid temperature.  The rate of heat transfer will depend on the fluid flow dynamics, fluid properties, pipe surface area, insulation, and the temperature difference between the process and ambient.  In order to maintain a desired fluid temperature, the amount of heat that is lost to ambient must be replaced by a heating system.  The ability and efficiency of a heating system to maintain the fluid temperature can be assessed by considering the heat transfer equation:

ΔT
The temperature difference is defined by the heating medium (steam, hot oil, etc.) temperature and the desired fluid maintenance temperature.  Using a hotter heating medium will increase the heat transfer rate.

U
The U value considers each component of thermal resistance within the system, including the ease with which the heating medium transfers heat to the pipe wall, conduction through the pipe wall, and convection from the pipe wall to the fluid.  Heating technologies differ in how easily they transfer heat from the heating medium to the pipe wall.  Jacketed piping offers the least resistance (and therefore highest U value), whereas tube tracing offers the most resistance.  The ability to achieve good contact between the heating element and the piping is critical in establishing the U value for a bolt-on jacketing or tube tracing heating system.

Heat transfer compound is often used with bolt-on jacketing and tube tracing systems to eliminate air gaps which can reduce the contact area between the heating element and the pipe.  A thin layer (1/16 to 1/8 inch) can be very effective since its thermal conductivity is 20X that of air.  However, heat transfer compound is not nearly as conductive as carbon steel (which is 100X more conductive), so thick layers of heat transfer compound can become ineffective.

Often, the ease with which heat is transferred from the pipe wall to the fluid is overlooked in the design of a heating system.  This is an important point since the fluid dynamics can be the limiting factor in heat transfer.  When a process is flowing rapidly through a pipe, it has the ability to accept heat more rapidly.  For processes where there is no flow (such as startup or melt-out scenarios), the heat that can be transferred into the process is significantly limited.  In these situations, the process rather than the heating system can become the limiting factor in heat transfer.

A
The heat transfer area is defined as the surface area in direct contact between the heating system and the pipe.  Jacketed piping, which completely encompasses the process pipe, offers the maximum area (and therefore maximum heat transfer).  For bolt-on jacketing elements and tube tracing, the area is defined by the size and shape of the elements.  A circular element will only touch the pipe at one location, but a rectangular heating element increases the contact area over a greater width.