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inductor is the area where the heating takes place, coil design is one of the most

important elements of the system and is a science in itself.

Definition:

The term "RF induction" is traditionally used to describe induction generators

designed to work in the frequency range from 100 kHz up to 10 MHz, in practical terms

however the frequency range tends to cover 100 to 200 kHz. The output range typically

incorporates 2.5 to 40 kW. Generally, the induction heaters in this range are used for

smaller components and applications such as induction hardening an engine valve.

The term "MF induction" is traditionally used to describe induction generators

designed to work in the frequency range from 1 to 10 kHz. The output range typically

incorporates 50 to 500 kW. Induction heaters operating within these ranges are normally

utilized on medium to larger components and applications such as the induction forging

of a shaft.

The term "Mains (or supply) frequency" is traditionally used to describe induction

coils driven directly from the standard a.c. supply. Most mains-frequency induction coils

are designed for single phase operation, and are low-current devices intended for

localized heating, or low-temperature surface area heating, such as in a drum heater.

Valve oscillator based power supply:

Due to its flexibility and potential frequency range, the valve oscillator based

induction heater was until recent years widely used throughout industry.[9] Readily

available in powers from 1 kW to 1 MW and in a frequency range from 100 kHz to many

MHz, this type of unit found widespread use in thousands of applications includingsoldering and brazing, induction hardening, tube welding and induction shrink fitting.

The unit consists of three basic elements:
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High voltage DC power supply

The DC (direct current) power supply consists of a standard air or water cooled

step-up transformer and a high voltage rectifier unit capable of generating voltages

typically between 5 and 10 kV to power the oscillator. The unit needs to be rated at the

correct kilovolt-ampere (kVA) to supply the necessary current to the oscillator. Early

rectifier systems featured valve rectifiers such as GXU4 (high power high voltage half

wave rectifier) but these were ultimately superseded by high voltage solid state rectifiers.

Self exciting class 'C' oscillator

The oscillator circuit is responsible for creating the elevated frequency electrical

current, which when applied to the work coil creates the magnetic field which heats the

part. The basic elements of the circuit are an inductance (tank coil) and a capacitance

(tank capacitor) and an oscillator valve. Basic electrical principles dictate that if a voltage

is applied to a circuit containing a capacitor and inductor the circuit will oscillate in much

the same way as a swing which has been pushed. Using our swing as an analogy if we do

not push again at the right time the swing will gradually stop this is the same with the

oscillator. The purpose of the valve is to act as a switch which will allow energy to pass

into the oscillator at the correct time to maintain the oscillations. In order to time theswitching, a small amount of energy is fed back to the grid of thetriode effectively

blocking or firing the device or allow it to conduct at the correct time. This so called grid

bias can be derived, either coactively, conductively or inductively depending on whether

the oscillator is a Colpitts, Hartley oscillator, Armstrong tickler or a Meissner.

Means of power control

Power control for the system can be achieved by a variety of methods. Many

latter day units feature thyristorpower control which works by means of a full wave AC

(alternating current) drive varying the primary voltage to the input transformer. More

traditional methods include three phase variacs (autotransformer) or motorized Brent ford

type voltage regulators to control the input voltage. Another very popular method was to

use a two part tank coil with a primary and secondary winding separated by an air gap.
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Power control was affected by varying the magnetic coupling of the two coils by

physically moving them relative to each other.

Solid state power supplies:

In the early days of induction heating, the motor-generatorwas used extensively

for the production of MF power up to 10 kHz. While it is possible to generate multiples

of the supply frequency such as 150 Hz using a standard induction motor driving an AC

generator, there are limitations. This type of generator featured rotor mounted windings

which limited the peripheral speed of the rotor due to the centrifugal forces on these

windings. This had the effect of limiting the diameter of the machine and therefore its

power and the number of poles which can be physically accommodated, which in turn

limits the maximum operating frequency.

To overcome these limitations the induction heating industry turned to the inductor-

generator. This type of machine features a toothed rotor constructed from a stack of

punched iron laminations. The excitation and AC windings are both mounted on the

stator; the rotor is therefore a compact solid construction which can be rotated at higher

peripheral speeds than the standard AC generator above thus allowing it to be greater in

diameter for a given RPM. This larger diameter allows a greater number of poles to be

accommodated and when combined with complex slotting arrangements such as the

Lorenz gauge condition or Guy slotting which allows the generation of frequencies from

1 to 10 kHz.

As with all rotating electrical machines, high rotation speeds and small clearances are

utilized to maximize flux variations. This necessitates that close attention is paid to the

quality of bearings utilized and the stiffness and accuracy of rotor. Drive for the

alternator is normally provided by a standard induction motor for convention and

simplicity. Both vertical and horizontal configurations are utilized and in most cases the

motor rotor and generator rotor are mounted on a common shaft with no coupling. The

whole assembly is then mounted in a frame containing the motorstatorand generator
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stator. The whole construction is mounted in a cubicle which features a heat exchanger

and water cooling systems as required.

The motor-generator became the mainstay of medium frequency power generation until

the advent ofsolid state technology in the early 1970s.

In the early 1970s the advent of solid state switching technology saw a shift from the

traditional methods of induction heating power generation. Initially this was limited to

the use of thyristors for generating the 'MF range of frequencies using discrete electronic

control systems.

State of the art units now employ SCR (silicon-controlled rectifier),[14]IGBT or MOSFET

technologies for generating the 'MF' and 'RF' current. The modern control system is

typically a digital microprocessorbased system utilizing PIC, PLC (programmable logic

controller) technology and surface mount manufacturing techniques for production of the

printed circuit boards. Solid state now dominates the market and units from 1 kW to

many megawatts in frequencies from 1 kHz to 3 MHz including dual frequency units are

now available.[8]

A whole range of techniques are employed in the generation of MF and RF power using

semiconductors, the actual technique employed depends often on a complex range of

factors. The typical generator will employ either a current or a voltage fed topology. The

actual approach employed will be a function of the required power, frequency, individual

application, the initial cost and subsequent running costs. Irrespective of the approach

employed however, all units tend to feature four distinct elements:[15]

AC to DC rectifier

This takes the mains supply voltage and converts it from the supply frequency of 50 or

60 Hz and also converts it to 'DC'. This can supply a variable DC voltage, a fixed DC

voltage or a variable DC current. In the case of a variable systems, they are used to

provide overall power control for the system. Fixed voltage rectifiers need to be used in

conjunction with an alternative means of power control. This can be done by utilizing a
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switch mode regulator or a by using a variety of control methods within the inverter

section.

DC to AC inverter

The inverterconverts the DC supply to a single phase AC output at the relevant

frequency. This features the SCR, IGBT or MOSFETS and in most cases is configured as

an H-bridge. The H-bridge has four legs each with a switch; the output circuit is

connected across the centre of the devices. When the relevant two switches are closed

current flows through the load in one direction, these switches then open and the

opposing two switches close allowing current to flow in the opposite direction. By

precisely timing the opening and closing of the switches, it is possible to sustain

oscillations in the load circuit.

Output circuit

The output circuit has the job of matching the output of the inverter to that required by

the coil. This can in it simplest form be a capacitor or in some cases will feature a

combination of capacitors and transformers.

Control system

The control section monitors all the parameters in the load circuit, the inverter and

supplies switching pulses at the appropriate time to supply energy to the output circuit.

Early systems featured discrete electronics with variablepotentiometers to adjust

switching times, current limits, voltage limits and frequency trips. However with the

advent ofmicrocontrollertechnology, the majority of advanced systems now feature

digital control.

The voltage-fed inverter:

The voltage-fed inverter features a filtercapacitoron the input to the inverter and

a series resonant output circuits. The voltage-fed system is extremely popular and can be
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used with either SCR's up to frequencies of 10 kHz, IGBT's to 100 kHz and MOSFETS

up to 3 MHz. A voltage-fed inverter with a series connection to a parallel load is also

known as a third order system. Basically this is similar to solid state, but in this system

the series connected internal capacitor and inductor are connected to a parallel output

tank circuit. The principal advantage of this type of system is the robustness of the

inverter due to the internal circuit effectively isolating the output circuit making the

switching components less susceptible to damage due to coil flashovers or mismatching.

The current-fed inverter:

The current-fed inverter is different from the voltage-fed system in that it utilizes a

variable DC input followed by a large inductor at the input to the inverter bridge. The

power circuit features a parallel resonant circuit and can have operating frequencies

typically from 1 kHz to 1 MHz. As with the voltage-fed system, SCRs are typically used

up to 10 kHz with IGBTs and MOSFETs being used at the higher frequencies.

STEAM:

The use of steam in the piping business is extremely common and the efficient handlingof steam lines is essential. The major consequence of running steam through piping is

that as the pipeline cools, the steam loses heat and produces condensate which if left in

the line will cause water hammer and subsequent damage to the piping system and/or

equipment.

STEAM PIPING:

All steam piping systems must be designed as follows:

Condensate must be removed from the steam line as soon as possible by the use of steam

traps and drip legs (Figure 8-3). Drip legs collect condensate and are located at all low

points in steam lines and at intervals in horizontal piping. A steam trap is connected to


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the drip leg and will open to allow the condensate to escape, then will close when it

senses steam thereby not allowing any steam to escape from the line.

Strainers must be provided upstream of the steam trap to prevent any scale or grit from

entering the trap and causing them to stick in an open position. Some traps have built in

strainers.

Typical Drip Leg

Steam Piping:

Typical Drip Legs at Header Ends

STEAM TRAPS


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The first function of any steam trap is to remove air and non-condensable gases from the

equipment to which it is assigned. If the air is not removed, steam will not be able to

enter the equipment. Hence, heat transfer will not occur.

Or, if air is not removed as designed, you may get uneven heating among different

components, poor steam distribution and possible corrosion.

The next job of the trap is to close in the presence of steam. There is a good reason for

this. For example, 1 lb of water at saturation conditions (15 psig and 250F) contains 218

Btu; 1 lb of steam at the same pressure contains 1,163 Btu. Of that, 945 Btu are in the

form of latent heat. That is to say, as the steam condenses into a liquid, it gives up its

latent heat you can see that much more energy can be removed from 1 lb of steam than

from 1 lb of water. You do not want steam to leave the system or process before it gives

up itslatent energy.

The last job of the trap is to drain condensate. As the steam gives up its latent heat, it

changes phase from a vapor into a liquid. This liquid is called condensate. This

condensate must be removed from the heat transfer equipment. If its not removed, then

you have less heat transfer area for the steam, and possible water hammer upstream of the

trap. Consequently, that means less heat will be transferred.

Steam Trap Purpose:

To remove condensate from live and exhaust steam lines, condensing equipment,

reboilers, heating coils, non self draining steam tracing manifolds and single tracers.

Method of Installation:

By utilizing drip legs to collect condensate and to extract same by steam traps.

Location:

a. At every low point in a steam system

b. If there is a long horizontal run of pipe, as in a piperack, then it will be necessary to

provide drip legs and steam traps at intermediate locations.

Steam trap suppliers such as Armstrong and Spirax Sarco are extremely knowledgeable

and offer excellent instructional information.

Steam Traps fall into three categories:

Mechanical Two Types - Ball Float, Inverted Bucket


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Thermodynamic - also called impulse or controlled disc

Thermostatic - temperature sensitive

Ball Float Valves:

On start up a thermostatic air vent allows air to bypass the main valve .

As soon as the condensate reaches the trap, the lever mechanism opens the main

valve. Hot condensate closes the air vent but continues to flow through the main

valve.

When all condensate is removed the float drops and closes the main valve, which

remains at all times below the water level, ensuring that live steam cannot be

wasted

Inverted Bucket Traps

The inverted bucket is the most reliable steam trap operating principle known.

The heart of its simple design is a unique leverage system that multiplies the force

provided by the bucket to open the valve against pressure. Since the bucket is open at the

bottom, it resists damage from water hammers, and wearing points are heavily reinforced

for long life.

Inverted Bucket Traps

On start up, upstream pressure raises the disc, and cooled condensate plus air is

immediately discharged.


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Hot condensate flowing through the trap releases flash steam at high velocity

which creates a low pressure area under the disc, drawing it towards the seat.

At the same time the pressure of the flash steam builds up in the chamber above

the disc forcing it down against the pressure of the incoming condensate until it

seats on the inner ring and closes the inlet. The disc also seats on the outer ring

and traps the pressure in the chamber.

Pressure in the chamber decreases as the flash steam condenses and the disc is

raised by the incoming pressure. The cycle is then repeated.

Thermostatic Traps:

There are two basic designs for the thermostatic steam trap, a bimetallic and a

balanced pressure design. Both designs use the difference in temperature between live

steam and condensate or air to control the release of condensate and air from the steam

line.

In a thermostatic bimetallic trap it is common that an oil filled element expands

when heated to close a valve against a seat. It may be possible to adjust the discharge

temperature of the trap - often between 60oC and 100oC.

This makes the thermostatic trap suited to get rid of large quantities of air and

cold condensate at the start-up condition. On the other hand the thermostatic trap will

have problems to adapt to the variations common in modulating heat exchangers

Thermostatic Trap


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Illustrations and text are taken from a Spirax Sarco website accessible or

related to


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insulation. Steam tracing is also used as an alternate to electrical tracing in freeze

protection of lines. In theory, the exact calculation of steam consumption is difficult, as it

depends on:

The degree of contact between the two lines, and whether heat conducting pastes

are used.

The temperature of the product.

The length, temperature and pressure drop along the tracer lines.

The ambient temperature. Wind speed. The emissive of the cladding.

Heated sampling point

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