Wednesday, November 25, 2015

Heat Transfer - The Basics

Hopper Heaters
Industrial hopper heaters are
example of conductive heat transfer.
Heat transfer is the movement of heat from one body or substance to another by radiation, conduction, convection, or a combination of these processes.

When heating a pan of water over a gas flame for example, all three forms a heat transfer are taking place. Heat from the flame radiates in all directions. Conduction takes place with the transfer of heat from the burner to the metal pan. This heat transfer is also responsible for making the handle hot after a period of time. Water is heated by the process up convection which is a circular movement caused by heated water rising and cold water falling.

The process of heat transfer also occurs when an object cools. If a mug of hot coffee is left standing on a cold kitchen countertop, its temperature will gradually decrease as heat is lost. The heat energy dissipates by conduction through the mug to the table top by convection as the liquid rises, cools and sinks, and by the radiation of heat into the surrounding air.

One way to conserve the heat a liquid and prevent heat transfer is to place it in a thermos. The use a vacuum chamber with silvered surfaces, along with low conductive materials, can greatly improve the amount of heat or cold that is lost to the surrounding environment. In between the silvered glass walls up a thermos lies a vacuum. In the case of a hot liquid heat transfer by convection through the vacuum is greatly restricted due to the absence have air molecules necessary to facilitate the transfer of heat. The lack of physical contact between the inside and outside walls of the thermos due to this airless space also greatly inhibits the movement of heat by conduction. Heat loss by radiation is prevented by the silvered walls reflecting radiant energy back into the thermos. Some conduction of heat through the stopper in glass can be expected but this too is limited because they are made of materials with very low conductivity. Thus the temperatures are both hot and cold liquids can be maintained by a properly designed thermos that limits the transfer energy through radiation convection and conduction.

Heat capacity is the amount of heat required to change the temperature of an object or substance by one degree Celsius. The heat capacity of water varies depending on its phase. As solid ice, the heat capacity of water is .5 calories per gram for every one degree Celsius, which means it takes half a calorie to raise the temperature of one gram of ice one degree Celsius.

As a liquid, water heat capacity is one calorie per gram for every one degree Celsius, so it takes one calorie of heat energy to raise one gram of water one degree Celsius.

The processes of phase change between solid, liquid, and gas also require a specific amount of heat energy. The amount of energy required to change a liquid into a solid, or a solid into a liquid, is known as heat of fusion. The amount of heat required to change one gram of ice to water is 80 calories. Similarly the heat vaporization is the energy required to transform a liquid into a gas. It requires 540 calories to change one gram of liquid water into a gas. With these values its easy to calculate exactly how many calories of heat energy are required to transform one gram a ice at absolute zero to steam.

To warm 1 gram of ice from -273 degrees Celsius to 0 degrees celsius would be 273 times .5 gram per calorie or about 140 calories. The phase-change of one gram a ice to liquid water requires 80 calories. Then to heat the water from zero degrees Celsius to 100 degrees Celsius with the heat capacity at one calorie per gram, would require 100 calories. The final phase change it one gram a boiling water to steam would require an additional 540 calories. Adding all of these values together yields 860 calories, the amount of heat energy it takes to transform one gram gram of ice at absolute zero to steam.

Watch this video for an illustration of the above:

Friday, November 20, 2015

Welding Preheat Basics

welding preheat
Why preheat welds?
Weld preheating is the process of heating the base metal (parts to be welded) to a specific temperature prior to welding. The specific temperature to which the part needs to be heated (before welding) is referred to as the “preheat temperature”.

The area requiring preheat may be the whole (entire) part, or just the area immediately surrounding the weld.

Preheating may continue during the actual welding process, but many times the energy generated from welding will be sufficient to maintain the desired temperature.  The temperature of the weld between the first pass and the last pass is referred to as “interpass temperature”. As long as it can be assured that interpass temperature will not fall below the preheat temperature, continued preheating is usually not required.

There are several key reasons why it's important to preheat before welding. First, a preheated part cools more slowly, which slows the overall cooling rate of the welded part. This improves the metallurgical (crystalline) structure and makes it less prone to cracking. Additionally, hydrogen that may be present immediately after a weld is also released more efficiently, which further reduces the possibility cracking. Preheating also mitigates stress from the shrinkage at the weld joint and nearby metal. Finally, pre-heating reduces the possibility of fracture during fabrication due to brittleness.

Electric welding preheaters, known as "ceramic mat heaters", are rugged and flexible heating elements designed so that they conform uniformly around the weld and surrounding area.  Ceramic mat heaters are normally controlled by a power console that uses thermocouples and electronic controllers to regulate, monitor, and many times record, the preheat temperature profile.

Welding code is the first determinant to whether pre-heating is needed. Welding code carefully specifies the minimum preheat temperature, the soak time, and the welding process. Many criteria are considered by welding codes, all gathered from years of rigorously tested data. This data is accumulated from many sources, including metallurgical science, chemical properties of materials, and radiographic analysis.

Determining whether or not preheating is required should not be taken lightly, as it is critical to the quality of a weld and therefore critical to the performance of a structure. When in doubt, review of industry code or contacting an industry expert, is imperative.

Friday, November 13, 2015

Automatic Heat Treatment Power Consoles

power console
Typical Power Console
(courtesy of HotfoilEHS)
Automatic heat treatment power consoles are used to control various heat treatment processes (i.e. pre-weld, post-weld) by closely controlling the temperature of the item being welded. The power console accurately controls the ramping rate (up and down), the soak temperature, the set point and the time. Power consoles are available from 2 to 24 zones of control. Zone can be used either in the fully automatic or manual mode.

ceramic mat heater
Ceramic Mat Heater
The power console is used to provide power to electric heating elements called ceramic mat heaters. Ceramic mat heaters are constructed of nichrome wire interwoven into ceramic beads which provides electrical insulation and protection. These heaters are quite rugged and conform to curved and irregular shapes.

Thermocouples are used to sense the target temperature and send their signal back to some type of electronic temperature controller, recorder, or combination thereof. The sophistication of the control system can range from simple manual control to fully automatic control with large graphic displays. Recorders are frequently used to document the pre-heat, soak, and post-heat process. Welding integrity depends on precise and accurate control.

Power Console Controller
Recorder used on
welding power console.
Heat treatment power consoles are built on sturdy chassis of steel and depending on ambient conditions, stainless steel. Construction includes wheels and handles for easy relocation and many electrical components for safety and convenience (such as amp meters, indicator lights, cut-off switches, fuses, and alarms).

For more information, contact:

Hotfoil-EHS, Inc.
2960 East State Street Ext.
Hamilton, NJ 08619
Phone # 609.588.0900
Fax # 609.588.8333
www.hotfoilehs.com
Email: dap@hotfoilehs.com

Thursday, November 5, 2015

Heat Tracing of Long Pipelines - Part Two


Part Two of Two Part Series

Installation of Heat Tracing


Heating tapes can be either “straight” traced or “spiraled”. Obviously, the easier method is straight traced.

Although heat tape can be supplied in unit lengths of several hundred feet, it is not advisable to have them this long. Long heaters are heavy and hard to handle and, if dropped or mishandled, fall into an uncoiled pile on the ground. To simplify installation and maintenance, medium lengths of heaters should be chosen, i.e. 150-300’-0”. Then, series junction boxes can be used to connect up the lengths of heaters to achieve the total pipe run.

On straight traced applications, the heaters must be secured at approximately 1’0” intervals to prevent sagging of the heater away from the pipe. Contact between heater and pipe is paramount. For heating tapes, securing fiberglass tape or similar should be used.

Junction Boxes
 
Normally on pipelines, there are three (3) types used:
  1. Voltage Supply Box. This is where the client’s supply is brought in and feeds the heating system. 
  2. Series Boxes. This is where “n” number are used to series connect the various lengths of heating means. 
  3. The back end box to connect the heaters in a star or “Y” fashion for three (3) phase applications. 
The boxes should be weatherproof for outdoor locations and suitable for any environmental attack from chemicals, gasses, dusts, etc.

Control of temperature can be achieved by a simple thermostat and contactor method, all the way up to sophisticated control panels. Each system of control must be investigated as to the requirements of the client/engineer for control, monitoring alarm levels, etc.

Repairs – fault finding

Fault finding on long, continuous circuits is very difficult. On uninterrupted runs of 1000’ or more with no series joints, unless there is mechanical damage, a break cannot be easily located. Where there are section lengths of 150-300’, it is easier to find the fault in such a section with standard electrical measuring instruments.

Above/below Ground Locations

The majority of heated pipelines are usually above ground. Some heated lines are below ground and, where such installations exist, records must be kept of the geographical routing, junction boxes, joints, etc. On underground lines, the thermal insulation must be totally waterproof as water tables do exist. Care must be taken on the installation due to the possible dissolved chemicals in the soil, which could attack the total installation.

Records must be kept of all systems, locations, items used, reference numbers of components, etc.

Hotfoil-EHS Design

On all long pipelines, the object is to reduce, to a minimum, the number of voltage supply points. By keeping these to a minimum, the cost of the total project of the heating system is attractive and competitive because it minimizes the electrical conduit and wiring.

Long pipeline systems usually need a three (3) phase voltage supply. Such a supply also offers a balanced three (3) phase load.

There are two (2) ways of achieving the requirements:
  1. A single, three phase heating tape (three foils in one sheath) 
  2. Three, single phase heating tapes (each tape with one foil) 
Although various sheaths can be used on the heating tapes, we have been using rubber. Silicone rubber offers many advantages, such as temperature range and chemical attack resistance.

Systems do not end with just the heating tapes. The junction boxes (series, supply and “Y”), must be provided. Also, the system has to be temperature controlled. For hazardous areas, the heating tape will invariably have to be braided.

Being a project engineering company, Hotfoil can supply all the accessories needed on any system and do all engineering designs, drawings, wiring diagrams, system layout, field supervision, startup services, etc.

Method (a) – One 3 Phase Heating Tape Hotfoil Type HTF – 3P

This system uses a single heating tape with three resistance foils as the heating means. (Sketch 2)

The foils can be of any material depending on the job requirement. As we are concerned with long lines, the foils are usually copper. Copper possesses a low resistivity, 10.3 ohms/c mil-ft. and thus long lengths can be achieved with this low resistance metal conductor.

Calculations are done to determine from loading needed (watts) with a given supply (voltage), the actual resistance of the circuit. This is then translated into the length and cross sectional area of the copper foil.

With the three (3) copper foils suitably spaced apart, they are fed through an extruder and receive a sheathing of silicone rubber. The thickness is dependent on the insulation factor of the project.

The back end of the tape system is taken through the leads to a junction box. On a 3 phase star/”Y” system, the three (3) leads are connected together to form a star point.

The front end of the system is connected to the voltage supply. This has to be a 3 phase supply. Since all three (3) foils are of the same cross sectional area and the same length, the load is balanced evenly over the 3 phases.

Typical systems done so far are:
  • One run of pipe/tape 5,300’ long, one supply point of 600 volts, 3 phase, giving a load of 5 watts per foot of tape/pipe. 
  • One run of pipe/tape 1,400’ long, one supply point of 208 volts, 3 phase, giving a load of 5 watts per foot of tape/pipe. 
  • One run of pipe/tape 7,920’ long, one supply point of 480 volts, 3 phase, giving a load of 7 watts per foot of tape/pipe. 
  • One run of pipe/tape 7,920’ long, one supply point of 480 volts, 3 phase, giving a load of 9 watts per foot of tape/pipe. 
These systems were for freeze protection of steam condensate return lines. The tapes use copper foils with silicone rubber sheaths. These were ideal as the rubber can withstand a 400° F continuous exposure, which the condensate could attain.

Also,
  1. One run of pipe/tape 1,780’ long, one supply point of 480 volts, 3 phase, giving a load of 7 watts per foot of tape/pipe. 
  2. One run of pipe/tape 850’ long, one supply point of 480 volts, 3 phase, giving a load of 7 watts per foot of tape/pipe.
Method (b) – Three Single Phase Heating Tape Hotfoil Type HTF – 1P

This system is basically the same as (a) but each tape is a single phase.

When systems call for high electrical loadings, both on the heating tapes and the pipes, or the pipe/circuit is exceptionally long, the foils must be of a larger cross sectional area. Due to this fact, individual foils are extruded with silicone rubber. (Sketch 3)

Extruded lengths of tape are kept to 100’-150’ due to the weight of the tape and the obtaining of foil in workable lengths.

Junction boxes are used for the series connections, star/“Y” connection and the incoming supply. The heating tapes are straight traced on the pipeline and secured with fiberglass or equal securing tape, every 1’-0”. Note: metal, plastic, nylon or pvc must not be used for securing due to mechanical damage or chemical non-compatibility.

Section lengths of tapes have cold leads, firmly butt spliced to the foils, and with a silicone rubber molding over.

The three (3) tapes are connected in a star/”Y” formation at the back end to achieve a balanced, 3 phase load.

A fourth redundant tape can be installed as a spare. Should any damage occur to one of the three working tapes, the fourth can be connected into the system at the series boxes quickly, and the heat is back on line. This means that the system is 100% operational without removing the thermal insulation or disrupting the system. When the pipeline is off line or shut down for other reasons, the repair of the damaged tape can be effected. This method of four tapes has been more than welcomed on many jobs.

Some projects done are:
  • 6,562’ run of pipe, 12” diameter to raise and maintain at 150°F. Most of the pipe was buried. 
  • 187,000’ of tape for pipes up to 36” diameter to raise temperatures and maintain up to 160°F. 
  • One run of pipeline, 6,853’-0” of 10” diameter, one 3 phase system, 67 KW, to maintain temperatures between 86° and 186° F, hazardous location. 
  • 118,000’of tape on 10” pipe with a total loading of 157 KW to maintain temperatures up to 104°F in a hazardous location.