WO2003005770A2

WO2003005770A2 - Electrical convective panel heater

Info

Publication number: WO2003005770A2
Application number: PCT/TR2002/000026
Authority: WO
Inventors: Erdinç SOY
Original assignee: Soy Erdinc
Priority date: 2001-07-02
Filing date: 2002-07-01
Publication date: 2003-01-16
Also published as: WO2003005770A3; AU2002321979A1; EP1410706A2

Abstract

An electrical convective panel heater which is used especially in automation panels and similar small cabinets for warming the environment as well as controlling the humidity therein. The body of the heater comprises air channels to provide air flow inside said body and allow for natural convection as well as forced convection through use of an optional fan. The heating element used in the panel heater has a sandwich structure comprising an indented middle plate around which resistance wires are wound in forward direction and also in the backward direction if a non-inductive heater is desired. The indented heater plate is further covered with at least two insulation plates of preferably mica laminate to provide better conductance.

Description

ELECTRICAL CONVECTIVE PANEL HEATER

Technical Field of Invention

Present invention relates to an electrical convective heater which is used especially in automation panels and similar small cabinets for warming the environment as well as controlling the humidity therein.

Background of the Invention

There are panels which monitor and control heavy industrial machines, various types of electrical equipments and instruments and general electrical installations of the plants in many fields of the industry. These panels generally contain a number of switches, relays, fuses and more recently various kinds of electronic equipments hosted within henerally a cabinet. Control panels, with their components as a whole, are not only used for commanding the electrical machinery/equipments but also providing safe operation of the electric installations of the system. This kind of control panels are usually located within the environment of the industry plant. As a direct consequence, the temperature and humidity rate of the environment, where the control or automation panel is installed, changes in parallel with the specific environment of industrial plant and also naturally with the place of the plant, the season and daily atmospheric conditions. On the other hand, electrical and electronical equipments within the panels which provide security and control, operate safely and effectively only within a narrow range as to the temperature and humidity rate. Further, total service life and reliability of these equipments may be recovered only under optimized service conditions.

As well known by those skilled in the art, electrical and electronical instruments are negatively affected by the humidity, salt and dust contained within the air and also the temperature of the environment. It is also not possible to expect the same performance characteristics of an automation panel when operating at 40°C in the summer and when operating at -20°C in the winter. Changes in the temperature are also naturally accompanied with changes in the humidity at wide ranges. Negative effects of subject changes directly result in safety risk on the plant and the machinery operated therein as well as reduction in the total service life of the control equipments installed within the control boards/panels. Apparently, the temperature and humidity of the cabinet in which the control panel is located shall be continuously controlled. Prior Art

Publications relating to the control of atmospheric conditions within the control and automation panels/cabinets are quite limited. Conventional systems used in practice are based on warming panel cabinets by passing electrical current over resistance wires. The most important disadvantage of such systems is that the resistance wires highly increase the surface temperature of the heater to such an extent that it is impossible to safely touch the surface of the heaters with naked hand after the heater is operated for some while. Furthermore, it is also known that heaters with high surface temperatures cause harm in various aspects to the equipments located nearby as well as to itself.. Humans who_. do need to work inside such cabinets or nearby the panels which are equipped with these types of heaters face the risk of burnt. A further disadvantage of these types of heaters is the magnetic field which forms around the resistance wires and inadvertently affects the electronic circuitry and equipments mounted on the control panels.

A different type of heater which is used for warming the control-automation panels is disclosed in EP application no: 87107420.9 of Eisenhauer et al. In this application there is described an equipment which produces heat by using PTC elements. Although the disclosed heater may be used for the purposes mentioned above, it offers an extremely expensive solution in parallel with its structure and the material. Additionally, excessive surface temperature on the disclosed heater accompanies danger as per described above.

Industrial Applicability The present invention is applicable in the field of electrical and electronical control and automation panels.

Brief Description of Figures

Fig. 1 is the upper cross-sectional view of a convective panel heater produced in accordance with the invention.

Fig. 2 is the schematic top view of the a heater plate within a convective panel heater produced in accordance with the invention.

Fig. 3 is the schematic perspective view of the aluminum body of the convective panel heater produced in accordance with the invention. Fig. 4 is the dismantled view of the sandwich heater plate of the convective panel heater produced in accordance with the invention. Fig. 5 is the cross-sectional view of the sandwich heater plate of the convective panel heater produced in accordance with the invention

Fig. 6 is the schematic top view of the indented plate of the sandwich heater plate of the invention.

Detailed Description of the Invention

The invention relates to a control-automation panel heater which comprises a sandwich heater plate placed inside a convective aluminum based body.

The aluminum based body (3) is in the shape of a rectangular prism which has inner channels that permit air flow in order to realize convective heating. The sandwich heater plate is placed inside a groove (5) which is preferably recessed in the mid-part of the body. Upon applying electric current onto the sandwich heater plate, the plate and the body (3) which is in direct contact with the plate, starts heating. The heated body (3) is preferably mounted vertically in the environment to be heated. Air flow occurs through the channels on the vertically mounted body. As the temperature of the air increases the air moves upwards through the channels and an air current forms inside the channels and a circulation around the body. This air movement may be in the shape of natural convection as well as forced convection. For the purpose of forced convection, a fan to be fitted at the bottom or top of the channels may be used. Said body (3) is in a preferred embodiment of the invention preferably produced of black eloxal aluminum whose heat transfer coefficient is fairly high. A clips may also be attached at the back side of the body (3) for easy-mounting onto bars inside cabinets.

Sandwich heater plate is an essential feature of the invention. Sandwich heater plate has a composite structure formed by winding a resistance wire (4) over an indented mid-plate (1) and covering the top and the bottom sides of said indented plate (1) with similar insulative plates (2. a, 2.b). As to forbid any current jump from the heater plate (1) onto the body (3), the width of the heater plate (1) which is widely wound with the resistance wire (4), is smaller than the width of other plates (2.a,2.b) at all locations where resistance wire (4) is wound. Said insulative plates (1, 2. a, 2.b) are preferably produced of mica laminate material in the preferred embodiment. Similarly, mica laminates are preferable as insulative plates for they provide perfect electrical isolation even at very high temperatures around 1000°C and they are also very easy to treat during manufacturing. Dielectric resistance of mica laminates are typically higher than 20 kV/mm. Insulation resistance of this material at 500°C is higher than 1x10¹² Ω x cm. Other types of materials which have good properties like mica laminates may also be used in the production of insulative plates.

In the preferred embodiment of the invention, resistance wire (4) is wound onto the indented heater plate (1) located in the middle of the sandwich structure. The parameters for selecting the resistance wire depends on the desired power of the heater. Heaters for control/automation panels disclosed by the invention operate roughly in the 0-440 V voltage range and 30-200 watt power range. The resistance wire (4) which is wound onto the mid-insulative heater plate (1), is preferably selected among Cr-AI-Si-Mn alloyed wires. The reason behind is the high temperature resistance of these even at 1500°C. Their resistances change generally in the 120- 150 mΩcm range. The properties and length of the wire which is to be used in winding the heater plate (1) can be determined after determining the desired panel heating power and the number of heaters.

Resistance wires (4) wound on the heater plate (1) of the invention shall be wound with constant pitch as to avoid any contact between each subsequent winding. The purpose is clearly to avoid any hot-spots originating from short-cuts and consequently any irregularity in the heating distribution. It has been found that a regular heating regime can be maintained by leaving at least 0.5 mm constant gap between each winding.

Sandwich heater plate shall preferable be fitted in a metal based body in order to achieve satisfactory convection. However, the risk in that is possibility of current jumps from the sandwich heater plate to the metallic body. A current jump may be in the form of either a physical contact between the resistance wire (4) and the metal body (3) or an arc in between the body and the resistance wire immediately in the vicinity of the body. A good insulation between the body and the sandwich heater plate shall be maintained for assuring elimination of such a risk. Otherwise, leakage current may be incurred on the control/automation panel which brings the risk of damage to the instruments on the panel as well as risk of electric shock to humans operating in the region of said automation panels. In order to eliminate this risk in the most cost-effective way, mid-insulative plate (1) of mica laminate material is at first cut in a rectangular shape. Thereafter, the rectangular laminate is further cut to indent at least 1 mm towards the centeriine which is parallel to the longer sides and the final shape shown in Fig. 6 is maintained. Resistance wires (4) are then wound over the indented narrow profile. This indentation eliminates the risks of a contact or an arc between the resistance wires (4) and the metal body (3) in the groove (5). As shown in Fig. 5, resistance wires (4) are wound around the indented heater plate (1) but they do not extend to the edges of the sandwich heater plate.

The resistance wire (4) is wound longitudinally around the indented heater plate (1) starting from the front part towards rear part leaving regular gaps between each subsequent winding. The rear end of the wire (4) remains at the front part of the plate (1) whereas the front end of the same wire is moved back to the front part of the plate following the end of last winding on the back part of the plate (1). The front end of the wire (4) then passes through a hole drilled on the front part of the plate (1) and brought next to the rear end of the wire. The rectangular bottom and the top insulating plates (2.a, 2.b) are then used to cover the upper surface and the lower surface of the indented heater plate (1) (Fig.4). The two ends of the resistance wire are then connected to two terminals attached to the front part of the panel (1). The plates (1, 2. a, 2.b) which are placed on top of each other are then riveted from their sides and the sandwich heater plate is formed. An additional rectangular insulative plate is placed on each faces of the sandwich heater plate and inserted inside the groove (5). The purpose of these additional insulative plates is to maintain a better contact with the groove.

The heater which is produced by winding the resistance wire starting from the front part and ending at the rear part also creates also creates magnetic field while heating the panel. However, some of the very sensitive electronical equipments which might have been installed on the control/automation panel may be negatively affected from this induction zone created by the windings on the heater plate (1). The solution suggested by the invention is to wind the resistance wire both in the backward and forward directions on the indented heater plate (1). A backward winding subsequent to an initial forward winding may be accomplished in the following sequence; the winding of the resistance wire (4) starts at the front part of the heater plate (1) and as it is reaches the rear end of the winding area, the wire is not passed to the lower face of said plate, the front end of the resistance wire (4), which was brought to the rear part of the plate, is wound around the plate in the opposite direction towards the front part of the plate (1), the front tip of the resistance wire (4) which reaches the front part of the indented heater plate (1) is attached to the terminal next to the second tip of the wire.

Opposite winding method helps to provide a non-inductive heater. Apparently, the magnetic field which is created by the first winding (i.e. forward winding) is in the opposite direction with the magnetic field created by the second winding (i.e. backward winding) and the two fields neutralize each other. The important point in preparation of the opposite winding is the fact that that all windings shall have enough clearance between each other such that no contact should be incurred between neighboring wires (4). To this end, the gap foreseen in the single direction windings shall be at least doubled in the initial forward winding in case a bi-directional winding is desired.

After inserting the finished sandwich heater plates inside the groove (5) in the body (3), a binder like silicon is added around the two ends of the heater plates in order to safely fix the plates in the groove (5). Insulating pipes are then dressed onto the terminals in order to finish the panel heater ready for electric appliance.

The panel heater is installed on the bars of automation/control panels through use of a connection clips located on the aluminum body. The product is, upon connection to a thermostat, ready for use in the control of the temperature and humidity within the panel cabinet. The operation of the heater may directly be controlled by means of a thermostat.

When compared with the similar products, the convective panel heater comprising a sandwich heater inside a convective body, is useful as to its low surface temperature eliminating the risks of damage to the electric-electronic equipments and burnt risk to the humans as well as to its low cost.

Claims

CLAIMS:

1. An electric panel heater suitable for convective heating of control/automation panels to control the temperature and humidity inside cabinets, said heater having a body comprising channels, characterized in that the heater plate comprises resistance wires wound around at least one laminated plate in a sandwich structure, said laminated plate of the sandwich structure is cut to have an indentation of at least 1 mm at the longitudinal winding locations which host the windings of the resistance wire, the resistance wire being wound starting from the first end of the winding area towards the second end around the heater plate with gaps such that contact of two subsequent winding is avoided in the entirety of the winding area, and the sandwich structure comprises at least two insulation plates covering the upper and lower faces of the indented heater plate.

2. An electric panel heater as set forth in Claim 1 characterized in that resistance wire is further backwards wound starting from the second end of the winding location towards to first end following the completion of initial forward winding such that the magnetic filed created by the forward winding is neutralized by backwards winding to provide a non-inductive heater.

3. An electric panel heater as set forth in Claim 1 or 2 characterized in that the heater comprises a fan which is suitable for providing forced convection over the channels of the body.

4. An electric panel heater as set forth in any of the preceding claims characterized in that the heater comprises a thermostat to keep the surface temperature of the body below 70°C.