Freeze Protection for Coal Handling Systems

Coal stored outdoors on the stockpile or delivered by unit train or barges picks up moisture from rain and snow. When this wet or frozen coal is conveyed, it inevitably comes into contact with the plate steel of the various hoppers and chutes within the coal handling system. During winter, this plate steel is below freezing for extended periods.

When wet or frozen coal encounters steel at sub freezing temperatures an instantaneous bond is formed. This bond causes immediate and often catastrophic blockage of the hopper and chutes. The bond and resultant blockage is so severe that pneumatic drilling equipment and explosives are often required to free up the system.

This problem, known as FLASH FREEZING, is extremely inconvenient and very costly. Several cases are documented where utility and industrial boilers have been shut down due to blocked conveying systems.

The document below describes effective electric heating systems for coal handling freeze protection.

Freeze Protection for Coal Handling Systems from Hotfoil EHS, Inc.

Transformer Basics

Electric transformer
(courtesy of Aftek EHS)

Transformers are composed of an iron core ring wrapped in coils. One coil is connected to an AC input voltage and is called the primary coil. The other coil is connected to an output circuit with the load resistance, and is called the secondary coil.

The two coils are well insulated from each other and do not form a physical electrical connection. This gives a transformer its unique electricity altering properties. Transformers can either step up or step down a voltage.

In a step down transformer, the number turns in the primary coil is greater than the number of turns in the secondary coil step up transformer the number of turns in the secondary coil is greater than the number of turns in the primary coil. The constantly changing current driven by an alternating voltage source induces a changing magnetic field in the core of the transformer.

The magnetic field created by the alternating current in the primary coil generates the flux in the transformer core. The secondary coil converts the flux back into current flow and produces a voltage at the load, or resistance, in the secondary circuit.

If there are fewer coil turns on the secondary then on the primary, this is called a step down transformer. The resulting voltage in the secondary circuit will be less than the primary.

In this example we have 20 turns on the primary coil and 10 turns on the secondary coil. To determine the decrease in voltage occurring in this step down transformer, we can use a simple ratio formula. This formula simply states that the secondary voltage to primary voltage ratio, is the same as the secondary coil to primary coil turn ratio. Rearranging the formula and then dividing 10 turns by 20 turns, we get .5 multiplied by 120 V. This results in a calculated step down voltage of sixty volts.

HofoilEHS - Your Preferred Source for Pre and Post Weld Heat Treating

HotfoilEHS is a premier manufacturer of pre weld and post weld heat treating equipment. EHS manufactures and sells pre and post-weld heat treating equipment including power consoles, ceramic mat heaters, thermocouple attachment units, pin welders, Brinell testers, Poldi hardness testers, thermocuple wire, insulation, and accessories.

The below video is a little shameless, self promotion.

Power consoles are standardly offered in 6, 9, 12, 18, and 24 zone configurations with a variety of control and recording systems.

Twin heating modules, with and without recorders, and optional ramping controllers are available from EHS.

For special projects, EHS can provide mineral insulated (MI) heating circuits designed into a blanket-type layout, using clips to attach the circuits to a wire mesh in a configured pattern. The circuits are CSA and FM Approved for hazardous and non-hazardous areas.

EHS also offers short or long term rentals on power consoles, temperature recorders, and ceramic mat heaters along with complete support and training services.

EHS engineers, designs, and manufactures proven systems to effectively complete any project more efficiently than competitive systems, while staying within budget. Years of application experience and successful installations have produced thousands of happy customers.

Ceramic Heater Pads for Weld Preheating and Post Weld Heat Treating

Welding ceramic pad heater
(courtesy of HotfoilEHS)

The goal of preheating and post weld (PWHT) heating is to maintain temperature between weld passes and to uniformly control changes in target temperatures. By properly achieving, maintaining, and then cooling weld temperatures, you lower weld stress, distortion, reduce shrinkage stress and allow unwanted hydrogen to escape.

It is very important to understand the material characteristics of the metals being welded, and know minimum and maximum preheat temperatures, particularly in tempered steels.

For most preheating, post heat treating, and interpass heating, precise temperature control isn't required. Its more important to maintain a minimum temperature, and stay within an acceptable range during the actual welding process, as well as during cool down.

Exceptions are with tempered steels. Tempered steels have already been heat treated at the steel mill and applying too much preheat can alter that tempering. In these cases, proper set point, temperature limiting, and temperature ramp rate of the welded part is critical.

Use of Ceramic Heater Pads

Resistance heating pads are constructed of ceramic beads strung on nichrome wire. These resistance heating elements accurately raise the workpiece temperature to the proper temperature before, during, and after welding, complying with recommended preheat, interpass, and PWHT practices. These semi-flexible ceramic heater pads, with their interlocking beads and high temperature wire, allow for a fit conforming to the shape of the workpiece, and are capable of temperatures up to 1,850 degrees F. Ceramic mat (pad) heaters have an additional benefit in that they don’t have to be moved during welding.

Recorders and controls used for
welding preheat and post heat.

Electronic temperature controllers use several thermocouples spot welded to the workpiece to monitor and regulate the actual part temperature throughout the operation. Many times the electronic controllers have ramping or temperature profiling capabilities, so that heat-up and cool-down can be carefully controlled. Recorders are often used to produce a record of the temperature profiles over time before, during, and after welding. This is important when welding jobs require careful documentation.

An average application is as follows: the heater pad is wrapped around the workpiece, and insulation is applied to the weld joint and the temperature controller is set. Once preheat temperature is achieved, a welder removes the insulation and starts their work. After the weld, the ceramic heaters can be placed over the weld and the controllers can be reset for proper PWHT.