Carbolite HTF User Manual

CONTENTS

Section page

Installation, Operation &

Maintenance Instructions

1700-1800°C Chamber Furnaces

types RHF & HTF

This manual is for the guidance of operators of the above Carbolite products

and should be read before the furnace is connected to the electricity supply.

Manuals for the furnace controller and overtemperature controller are

supplied separately.

Please read the controller manuals before operating the furnace.

MF22 – 3.09

Instrument switch: when the instrument switch is operated the temperature
control circuit is energised.

Heat Light: the adjacent light glows or flashes to indicate that power is
being supplied to the elements

DANGER of electrical shock– read any warning printed by this symbol.

DANGER – hot surface. Read any warning printed by this symbol.
WARNING: all surfaces of a furnace may be hot.

DANGER – read any warning printed by this symbol.

RHF & HTF

1.0 SYMBOLS & WARNINGS

1.1 Switches and Lights

1.2 Warning Symbols

2 MF22 3.09

RHF & HTF

separately packed

items

RHF 17/25
HTF 18/15

HTF 18/27

Elements 6

9

Element clamps

12

18

Element clips

12

18

Braids 1 set

1 set

Separators/Blocks

6 9

Chimney unit

1 1

2.0 INSTALLATION

2.1 Unpacking & Handling

When unpacking or moving the furnace always lift it by its base. Never lift it by the door. Even the

smaller models contain transformers and are heavy: use two or more people to carry them.

Remove any packing material from the door gear and from inside the furnace chamber. The door

can be lifted manually and supported open, but in the electric door models it is not
counterbalanced. Close the door with care to prevent any damage to insulation surfaces; or for
electric door models, await electrical connection and close the door electrically.

NOTE: This product contains Refractory Ceramic Fibre (better described as Alumino Silicate

Wool) for precautions and advice in handling this material see the ‘Repairs and Replacements’

section.

2.2 Siting & Setting Up

Place the furnace in a well ventilated room, away from other sources of heat, and on a surface

which is resistant to accidental spillage of hot materials. Do not mount the furnace on an
inflammable surface. Do not mount the furnace where radiated heat from an open door could cause
damage.

Ensure that there is free space around the furnace. Do not obstruct any of the vents in the case:

there are cooling fans in the case which must not be obstructed.

Ensure that the furnace is placed in such a way that it can be quickly switched off or disconnected

from the electrical supply - see below.

2.3 Fitting the Heating Elements

Wear eye protection when handling the heating elements. See the warning in section 5.6.

The Molybdenum Disilicide elements are EXCEPTIONALLY FRAGILE and are packed separately,

together with other items as shown in the list.

Remove the top access panel to allow element fitting.
Fit the heating elements vertically into the chamber. Handle the elements with great care. The

installation of these elements is described in section 5.6. Connect the aluminium braids securely as
indicated in the element installation instructions.

MF22 3.09 3

CONNECTION DETAILS

supply type

Supply

Terminal label

Cable colour

Live-Neutral

Reversible or Live-Live

1-phase

L

Brown

To live

to either power conductor

N

Blue

To neutral

to the other power conductor

PE

Green/Yellow

To earth (ground)

to earth (ground)

supply

Terminal label

Cable colour

3-phase

L1

Black

to phase 1

L2

Black

to phase 2

L3

Black

to phase 3

N

Light Blue

to neutral except delta

PE

Green/Yellow

to earth (ground)

RHF & HTF

2.4 Electrical Connections

Connection by a qualified electrician is recommended.
The smaller models are designed only for single phase electrical supplies with or without neutral.

The larger models are for 3-phase use, but can also be supplied for single phase. The furnace must
be connected only to the type and voltage of supply for which it was ordered.

The voltage or range of voltages on which the furnace may be operated is given on the furnace

rating label. Check that the supply voltage is compatible with the voltage on the label, and that the
current capacity is sufficient for the amperage on the label, before connection to the supply. A
table of the most common ratings is given in section 8.1 of this manual.

In some models supply cables are fitted, as are internal supply-rated fuses. The cable may be

directly wired to an isolator or fitted with a line plug.

The supply point must be within reach of the furnace operator and must incorporate either an

isolating switch which operates on both conductors, or a quickly removable plug.

The supply MUST incorporate an earth (ground).

DO NOT connect a furnace ordered for three phase use to a single phase supply or to the wrong

type of three phase supply.

4 MF22 3.09

RHF & HTF

3.0 OPERATION

The instructions for operating the temperature controller are given in a separate manual.
If the furnace is fitted with a time switch, see also the supplementary manual MS03.
If cascade control is fitted, see the supplementary manual MS07.

3.1 Operating Cycle

The furnace is fitted with an Instrument switch. The switch cuts off power to the controllers and

contactor.
Connect the furnace to the electrical supply. The cooling fans automatically operate.
Electric door models: before switching on the control instrumentation, check the door switch for

correct operation. The spring return switch stops the door in any position, but over-travel is

automatically limited. Load the furnace and close the door.
Manual door models: observe the warning in 2.1 about gentle operation of the door.
Operate the instrument switch to activate the temperature controller. The controller becomes

illuminated and goes through a short test cycle.
Close the furnace door and adjust the temperature controller – see the controller manual.
Set the overtemperature controller to a temperature a little (say 15°C) above the maximum setpoint

of program temperature, according to the instructions in the appropriate manual.
Unless a time switch is fitted and is off, the furnace starts to heat up. The Heat light glows brightly

at first, more dimly as the furnace temperature approaches a program setpoint.
If the overtemperature trip operates then an indicator or message in the overtemperature controller

flashes, and the heating elements are isolated. Find and correct the cause before resetting the

overtemperature controller according to the instructions supplied.
To switch the furnace off, set the Instrument switch to off. The case cooling fans remain on and

the chamber can be unloaded. Leave the fans on until the furnace cools to below 300°C. If the

furnace is to be left off unattended, isolate it from the electrical supply.

3.2 General Operating Advice

Heating element life is shortened by use at temperatures close to maximum. Do not leave the

furnace at high temperature when not required. The maximum temperature is shown on the

furnace rating label and on the back page of this manual. The furnace can be cycled between room

temperature and maximum without a detrimental effect on element life.
On first installing the elements, and on subsequent element replacement, run the furnace at 1500°C

for an hour to create a protective glaze on the element surface.
When heating large objects, in particular poor conductors, avoid shielding the thermocouple from

the heating elements. The thermocouple is intended to sense the temperature near the heating

element, but if a large object is placed in the chamber it may record the average temperature of the

object and the elements, which can lead to overheating of the elements. Allow large objects to gain

heat at a lower temperature and then reset the controller to a temperature close to the desired

maximum.
The furnace elements are very susceptible to mechanical shock. Take great care when loading or

unloading the furnace chamber. If it is necessary to load or unload work at elevated temperatures,

keep the door open for as short a period as possible. The insulation cools quickly and may crack

through thermal shock. Note, though, that the insulation material is susceptible to some surface

cracking arising from high temperature cycling; such cracking is not detrimental to the

performance of the furnace.
Lightweight ceramic fibre insulation can easily be marked by accidental contact. Some fine cracks

may be visible on the surface of the insulation, or may develop in the surface of the chamber due

MF22 3.09 5

RHF & HTF

to the progressive shrinkage of the insulation materials. Cracks are not usually detrimental to the

functioning or the safety of the furnace.

3.3 Atmospheres & Corrosive Materials

In oxidising atmospheres, metal oxides react with the silica layer on the surface of the elements

and may lead to premature failure. Protect the elements from splashes of molten metal and dust

when charging the furnace, and from fumes developed when melting, especially from fluxes. Also

avoid compounds with a high alkali content.
The furnace is designed for use up to its maximum temperature in oxidising atmospheres, but can

be operated successfully in neutral or carburising atmospheres. It may be used with nitrogen, argon

or helium atmospheres to 1600°C. Reducing atmospheres are not recommended. Sulphur dioxide

in normal concentrations is not harmful, but chlorine, and more so fluorine, strongly attack even

oxidised elements and should be avoided.
The furnace is not recommended for burning off carbonaceous materials. Other Carbolite furnaces

are available for this application.
When an optional gas inlet is fitted there is a label near the inlet saying "INERT GAS ONLY". In

practice inert or oxidising gases may be used, but not combustible or toxic gases. Chamber

furnaces are not gas tight, so it should be understood that gas usage may be high, and that the

chamber is likely always to contain some air. Residual oxygen levels of 1 % are to be expected.
The hearth can be protected from abrasion, if required, by the provision of a secondary plate. This

consumable item can either be in light weight ceramic fibre or in dense, hard-wearing alumina

refractory.

3.4 Operator Safety

The furnace incorporates a safety switch which interrupts the heating element circuit when the

door is opened. This prevents the user touching a live heating element, but also prevents the

furnace from heating up if the door is left open.
Avoid burns. Carbolite can supply tongs, face masks, and heat resistant gloves. Before you remove

a hot object from the furnace make sure you have a safe place to put it down.
When the door of a hot furnace is opened there is considerable radiative heat. Do not keep any

inflammable objects near the furnace.
See also the warning about eye protection in section 5.6.

3.5 Note on Temperature Control

The furnace is designed for rapid heating and cooling applications The programmer enables the

furnace to heat or cool at slower rates as desired, and variable "hold" (dwell) periods can be

programmed as required.
The programmer is used in conjunction with a phase angle thyristor Eurocube, which incorporates

a current limit potentiometer preset by Carbolite. The models designed for 3-phase use have three

of these thyristors.
The elements are connected in series across the low voltage output of a transformer housed in the

furnace case. Molybdenum disilicide elements do not age, so if an element fails it is not necessary

to replace the complete set.

6 MF22 3.09