Engineer Talk

​Grounding is often treated as an afterthought, but not all grounds or ground connections are the same. Too often we just assume that any ground connection will be a ‘good’ one. Grounding a device generally serves two purposes. The first is to provide a safe path for current flow in the event of a short circuit. The second is to provide a true “zero voltage” reference for the circuit.

What's the difference?

The majority of Tutco – Farnam Custom Product’s heaters are used to heat an air flow. When determining the power required to heat a stream of air, invariably two questions will come up.

1. How much “stuff” do we have to heat up? In more technical terms, what is the mass flow rate?

2. How hot does the air have to get? In other words, what is the temperature rise required?

We are going to concentrate on the first question. In the US, it is very common to see the volumetric air flow rate stated in units of Cubic Feet per Minute. Whether it is supplied by a fan, blower, or compressed air line, the volumetric air flow rate is simply stating the volume of air passing through over a given unit of time. SCFM is an acronym for Standard Cubic Feet per Minute, with “Standard” implying standard conditions. For the sake of this discussion, we are going to assume standard conditions of 1 atm and 68°F . Different industries and standard organizations actually cite slightly different values for what standard conditions are, but we are going to save that discussion for another time.

Once process equipment is designed and placed into service, the importance of each individual component is frequently forgotten. Unfortunately, heaters often find themselves in this situation until there is a failure. Since heaters usually play an integral role within the application, any failure can rapidly expand to a major issue or shutdown. The good news…many of these disasters can be avoided by increasing the mean time between failures!

​Thermodynamics, specifically heat transfer, is used throughout our daily lives, but not always thought of.  A common practice of cooking breakfast would be one simple example.  You place one type of media, your frying pan, onto a hot surface and apply the “heat”, which is your source of energy, to cook the food.  The heat transfer that is occurring between the higher temperature stove-top and the cooler frying pan is a great practical application … 

​When designing an electric heating system for industrial processes, many factors must be addressed. These factors routinely include required power, location, ducting, air source and controls, but circuit protection is often overlooked.  Proper circuit protection is of utmost importance not only for safety, but also to avoid costly down time and repairs to the system.  Electrical branch type and size of protection are important considerations when adding circuit protection to a heating system.  Depending on the type of controls employed speed of operation can also be a factor.

As the name implies, duct heaters are generally designed to be installed into ducting. They are usually installed through the side wall to cause air in the duct to he heated as it flows around and through the open-coil elements. Duct heaters made by Farnam are not for HVAC use. They are for industrial type applications and are not built to the standards required for the typical residential HVAC. For HVAC applications go to www.tutco.com.

In the heating industry we are often asked what is the difference between a distributed wattage heater and a zoned heater. These various construction techniques can be applied to many different types of conduction heaters such as cartridges, ceramic and mica strips, and band heaters. Almost any heater that uses a wound coil can use one of these construction methods of controlling watt density. Before beginning, some definitions should be discussed.

Sometimes, when all the design work is done, the heating element is too fragile for either the assembly process, or field operation. This can occur when making a low wattage heater that uses a high voltage. In these cases the element needs to be more robust but the design won't allow it. One solution is to use a diode in series with the heating element. The diode will reduce the applied energy to the heater for any given voltage and allow a larger lower resistance heating element to be used. Naturally this is not applicable to DC powered heaters.

By following a few rules-of-thumb you can determine the wattage requirement for your application.

Calculating the wattage requirements to heat a system is a straightforward process as long as all the parameters of heat energy flowing in and out of a system are considered. Heat requirements that must be considered are:

• Initial heat for startup of the system, usually from an ambient temperature to a desired processing temperature.
• Heat losses to the environment due to conduction, convection and radiation.
• Heating of material being processed during operation.
• Heating of material flowing through the process such as a liquid that will be heated and pumped to be used elsewhere.
• Losses due to phase changes of materials, either during initial heatup or while processing (melting a solid to a liquid or boiling a liquid to a gas).

Overview

Watt density is a useful measure when considering the various types of heating elements available. Tutco-Farnam Custom Products manufactures open coil elements which are generally used to heat a gas flow, such as air or nitrogen. Tutco-Farnam Custom Products also manufactures surface heaters which heat by conduction. This discussion will pertain specifically to watt density and open coil elements.

What is Watt Density?

Watt density is defined as the heating element power divided by the actively heated surface area of the element. If you were to pick up a heat transfer textbook, power/surface area is identified as the heat flux, but the term watt density has found common usage within the industry. In the United States, the units commonly used are [Watts]/[inches]2. In other countries, [Watts]/[mm]2 is often seen.