GB2550157A

GB2550157A - A UV Discharge lamp with improved operating life

Info

Publication number: GB2550157A
Application number: GB1608178.8A
Authority: GB
Inventors: slater Mark
Original assignee: Alpha-Cure Ltd
Current assignee: Alpha-Cure Ltd
Priority date: 2016-05-10
Filing date: 2016-05-10
Publication date: 2017-11-15
Anticipated expiration: 2036-05-10
Also published as: GB201608178D0; GB2550157B; WO2017194930A1

Abstract

A UV discharge lamp with improved operating life features a sealed chamber 12 and two pairs of electrodes 13, 14. A running electrode pair 13 is configured to run at a relatively high operating current, compared to a standby electrode pair 14 having a lower operating current. The running electrodes 13 are activated during normal operation when the lamp is required to emit a UV source for use in an industrial process. The standby electrodes 14 are activated, and the running electrode deactivated, during a standby mode in order to maintain a "ready-to-use" state. In practice the service life of the lamp is extended because each electrode pair is run closer to its optimum temperature, reducing a sputtering effect which is detrimental to operation and ultimately leads to failure.

Description

A UV DISCHARGE LAMP WITH IMPROVED OPERATING LIFE

The present invention relates to a UV discharge lamp with improved operating life, particularly the utilisation of two dissimilar pairs of electrodes in a medium pressure UV discharge lamp, operable over an extended power range.

Background to the Invention A medium pressure (MP) UV discharge lamps' operation relies on an electrical discharge (or arc) forming between two electrodes supported in a fused silica (quartz) envelope. This envelope contains the chemicals that are required for initiating/sustaining the arc and also for producing the desired wavelength of UV radiation.

The electrical discharge is initiated by applying electrical energy to the electrodes such that it causes them to be of opposite polarity, resulting in the gases inside the envelope being ionised. During this process electrons emitted from the tip of one electrode flow across the lamp to reach the opposing electrode. MP lamps typically utilise electrodes manufactured from Tungsten which is a refractory metal suitably resistant to wear and temperature, since it has the highest vaporisation temperature (boiling point) of all the elements. Nevertheless, the constant high energy electron emission occurring in MP lamps removes material from the emitting surface in a process called 'sputtering'.

This sputtering effect is greatly affected by the electrode operating temperature. The increase in electrode temperature predominantly comes from the conversion of electrical energy in the gas discharge and opposition to the electron flow as it passes through the electrode (electrical resistance), both of which are directly affected by the amount of electrical current flowing through the lamp.

Generally, the end of life failure indication for MP lamps is that sputtered tungsten becomes deposited on the inside of the lamp envelope. This directly blocks the required radiation from leaving the lamp (and hence is no use to the external process the lamp is required for) and also chemically combines with other material essential for the efficient production of UV wavelengths. Tungsten deposits also lead to a higher absorption of radiated energy in these areas, increasing the envelope wall temperature and accelerating other failure modes. From the foregoing it can be seen that optimising electrode operation is essential in achieving an acceptable lamp life.

Operators of MP lamps are primarily interested in the maximum power output of the system in order to achieve the necessary process use requirements. In broad terms the lamp output power is the product of the operating voltage and current available. Therefore, to achieve the desired specific lamp power, the lamp designer has to know the current available from the power supply to be used and then designs the lamp operating voltage accordingly.

It is impractical for electrodes to be designed for individual operating currents, so they tend to be operable over a range of currents. It is down to the knowledge of the designer to determine the correct specification.

It is common practice for designers of MP lamps to select an electrode based on the maximum lamp current because, otherwise, overheating will occur, therefore it operates closer to the vaporisation temperature where the amount of material sputtered from the electrode is greatly increased. However, if a high current electrode is selected and the operating current is too low then the electrode is operated in an overcooled state. This changes the mechanisms for the emission of electrons and, again, results in more electrode material being sputtered into the lamp. Deviations in lamp current from a nominal value in both directions can therefore cause blackening of the lamp envelope resulting in a reduction of UV transmission through to the process. MP UV lamps are being increasingly operated using electronic power supplies which usually have a lower open circuit voltage than traditional wire wound power supplies. The lower open circuit voltage results in increased running current to achieve a similar lamp power density. The increased operating current typically means that electrode sizes have had to be increased to stay within acceptable electrode running temperatures to ensure good lamp life.

Discharge lamps do not have the ability to be turned off and on quickly due to difficulty in reinitiating the discharge whist the lamp is hot from previous operation. Therefore, it is common practice to run lamps in a 'standby' setting achieved by reducing the current supplied from the power supply. This allows the intensity of the lamp to be changed rapidly to accommodate the process without the issue of restarting a hot lamp.

As system designers strive to use less total energy there is a tendency to reduce the 'standby' power of the lamp by significant amounts, for example in some cases as low as 20% of the maximum lamp current. This has the benefit of effectively reducing the duty cycle of the total system resulting in less energy to be consumed overall. However, the reduced 'standby' power results in the electrodes within the MP lamp electrode being run in a cold state. This practice will shorten the lamps useful life, depending on the amount the current is reduced by and the operating time at this level.

Despite best efforts, the general degradation of UV (and visible) output from a lamp is shown in Figure 1. A steady decline of percentage output (100% being unimpeded output) can be seen over time, which becomes significant after several thousand hours of operation.

Summary of the Invention

The present invention seeks to address problems observed in the prior art, for the purposes of improving MP lamp operating life.

In broad terms according to the invention a medium pressure lamp fitted with four electrodes, instead of the usual two, can be operated in a way that prolongs the life of the lamp. The invention is broadly defined according to the appended claims. Particularly, one pair of electrodes is selected to run at the maximum operating current as determined by the UV system and power supply (hereinafter referred to as "running electrodes") while another pair of electrodes is selected according to the smallest operating current specified for the standby setting (hereinafter referred to as "standby electrodes"). Each end of the MP lamp requires one of the pair of running electrodes and standby electrodes respectively. These two electrodes are electrically isolated from each other by a dedicated connection through the lamp seal.

The advantage of the invention is that each pair of electrodes will be subjected to a smaller current variation, keeping within a more comfortable operating window for respective designs. Accordingly, the invention will result in each electrode pair being run closer to their optimum temperature, reducing the sputtering effect and therefore extending the useful operating life.

Brief Description of the Drawings

Figure 1 shows a graphical representation of UV degradation throughout service life as observed in the prior art; and

Figure 2 illustrates an end of a UV lamp made according to the invention.

Detailed Description of a Preferred Embodiment of the Invention

Figure 2 illustrates a section view of an end of a UV lamp 10 with an envelope wall 11 forming a sealed chamber 12 containing gas in the known way. The wall 11 is preferably formed from fused silica, although any suitable material could be contemplated. According to the invention, electrodes 13 and 14 protrude through a distal end of the sealed chamber 12, set into the silica material. The larger electrode 13 corresponds to a running electrode that is preferably selected to run at the maximum operating current as determined by the UV system and power supply, while smaller electrode 14 corresponds to a standby electrode chosen according to the smallest operating current specified for the standby setting. It will be apparent to a skilled person that the electrodes 13/14 are paired with a matching electrode located at an opposite end of the lamp 10. This would appear as a mirror image of the view illustrated by Figure 2. The construction of an electrode is generally known in the art and it will be apparent that the "business ends" 15/16 of the respective running and standby electrodes 13/14 are located within chamber 12 where an arc can be formed across the lamp, toward a second end (not seen in Figure 1). The electrodes are electrically connected to an external device mounting (not illustrated) via terminals 17/18 respectively. The mounting will be part of an apparatus including a control system for operating the lamp. In practice there will likely also be a ceramic cover protecting each end of the lamp to assist mounting in the UV control device/power supply (not shown).

According to practical implementation of the invention it is likely that the running electrode pair 13 would be activated at operating currents above a certain threshold, for example when the current rises above 50% of the designed tolerance, whereas the standby electrode pair 14 would be activated below this threshold. However, alternative configurations are possible following experimental testing and optimisation of the inventive concept.

The invention requires the means, within a control system, to switch the current flow between the respective pairs of electrodes, in line with the lamp operating current required at that time. The benefit of such an arrangement is that each pair of electrodes will be subjected to a smaller current variation, keeping within an optimum operating window for respective designs. Accordingly, the invention will result in each electrode pair being run closer to their optimum temperature, reducing the sputtering effect and therefore extending the useful operating life.

The advantageous aspects of the invention are particularly important when fitted into a UV lamp system that has very low standby power settings or when the process requirement dictates longer periods of time with the lamp being run in a standby mode.

It will be apparent that a UV lamp according to the invention can be constructed from available materials, e.g. Tungsten electrodes and fused silica chamber, and manufacturing techniques, so long as the provision is made for a second pair of electrodes suitable for standby mode use. As described above, the invention is embodied by a UV lamp and/or system comprising same, where a running electrode is configured to run at a relatively high operating current, compared to a standby electrode having a low operating current. The running electrode is activated to form an arc and ionise gas within the chamber in the known way in order to create a source of UV wavelength emissions for use in an industrial process. The standby electrode is activated (and the running electrode deactivated) during a standby mode in order to maintain a "ready-to-use" state. In practice the service life of the lamp is extended because each electrode pair is run closer to its optimum temperature, reducing the sputtering effect detrimental to operation.

Claims

Claims:

1. A UV lamp comprised of: a sealed chamber, containing gas for ionisation; a first pair of electrodes extending through a wall at opposed ends of the sealed chamber; a second pair of electrodes extending through a wall at opposed ends of the sealed chamber; wherein the first and second pairs of electrode have different optimum operating characteristics.

2. The UV lamp of claim 1 wherein the first electrode pair corresponds to a running electrode that is selected to run at a relatively high operating current and the second electrode pair corresponds to a standby electrode selected to run at a relatively low operating current compared to the running electrode pair.

3. The UV lamp of claim 1 or 2 wherein the first electrode pair and second electrode pair are located in parallel through the wall of the sealed chamber, each with electrically isolated terminals for electrical connection exposed outside of the chamber.

4. A UV lamp system incorporating a UV lamp according to any preceding claim including a switch means for activating/energising the first and second electrode pairs independently and/or alternatively as needed.

5. The UV lamp system of claim 4 wherein the switch means is managed by a control means.

6. The UV lamp system of claim 5 wherein the control means causes the switch means to activate the first electrode pair when the lamp is operating above a defined threshold current and activate the second electrode pair when operating below the defined threshold current.

7. The UV lamp system of claim 6 wherein the defined threshold current is 40% - 60%, preferably 50%, of the maximum operating current rating of the first electrode pair.