Patent Grant
8459839
The invention relates to portable, moderately high power, ultraviolet lamps.
Beams of high intensity ultraviolet (UV) light are useful for curing polymers in coatings, inks, adhesives and the like, and for other purposes. A known reliable source of UV light at good power is the mercury vapor lamp. Almost all of the world's artificial lighting comes from mercury lamps, mainly due to their efficiency and low cost. For the same reason, mercury lamps have been the staple of the UV curing industry for many decades.
Mercury lamps powered by heavy transformer ballasts, once used only in factory installations, are now appearing in on-site UV curing applications. New waterborne and dual-cure UV coating technologies are also expanding UV curing from factory to the jobsite. Bathtubs, countertops, floors, walls, etc., are now finished and re-finished quickly in the field, with conventional curing equipment. Lighter, simpler, and more practical needs along with recent economic and environmental concerns are driving the demand for more portable lower cost UV systems. Lower cost and portability will drive the equipment side of the UV curing market, while appearance, durability, and applicability will dominate coatings.
A ballast circuit is used with gas or vapor discharge lamps to control their power usage and prevent runaway overloading of the power supply. Ballasts can include resistors, capacitors, inductors, transformers or a combination of these, as well as electronic circuits where the power to the lamp is controlled by high frequency switching techniques. Today the most widely used and heaviest part of the ballast for the lamps in UV curing equipment is the transformer.
While ordinary fluorescent lamps for lighting need only small ballasts due to their low power, typical UV curing lamps use tens or hundreds of watts per centimeter of length, requiring high-power ballasts rated at 1 to 40 kW. Magnetic transformer ballasts use iron cores. Their weight in kilograms (kg) is approximately 5.4 P+18, where P is the power in kW. The most common power source is the 120 volt 15 Amp residential outlet delivering a maximum of 1.8 kW. Other electrical outlets deliver 220 volt at 15, 20, 30 and 50 amps, respectively delivering a maximum of 3.3 kW, 4.4 kW, 6.6 kW, and 11 kW of power to the lamp. Typical UV lamps operating at these power levels require magnetic ballasts that will weigh approximately 28, 36, 42, 54, and 77 kg, respectively. The entire system, however, can weigh more. This limits the portability of the UV curing equipment.
Electronic ballasts offer an alternative to the heavy magnetic ballast. 3.5 kW compact electronic ballasts are now available in the marketplace weighing less than 7 kg, but their cost is significantly higher than their magnetic ballast counterparts. Resonant and hybrid technologies, offer equipment at more moderate weights and prices.
In a prior handheld UV curing device of the present inventor, the electrical ballast is a resistance wire (e.g., of Nichrome®). The resistance wire also doubles as thermal ballast for the lamp. Air from a fan is blown across the wire in a path that takes the air past the lamp. A curved elongated optical reflector associated with the lamp is split so that air can enter a plenum defined by the reflector wherein the lamp is mounted axially. Whenever the lamp is cold, air heated by passing over the resistive wire of the ballast heats the lamp toward its operating temperature. When the lamp temperature exceeds the temperature of the wire, the airflow then acts to cool the lamp. This arrangement tends to stabilize the lamp's thermal performance, and since the voltage across an electrical discharge is dependent upon the gas pressure and thus upon the lamp's temperature, the arrangement also stabilizes the lamp's electrical performance.
For UV outputs of over 40 W/cm with beam widths up to 12 cm and at a curing distance of 3 to 5 cm, large housings are used to provide room for the UV lamp, ballast circuitry, and any cooling structures. What is needed is a lightweight ballast circuit for a moderate power UV lamp that can be used in a portable curing apparatus. A portable, even handheld, device would offer both speed and precision for curing of polymer coatings on surfaces of all shapes.
An ultraviolet beam generating apparatus is provided with variable-resistance ballast electrically connected to one or more mercury vapor discharge lamps in a reflective lamp housing. The resistance is selected as a function of lamp voltage to counterbalance the decreased resistance within the lamp discharge as operating power and temperature increases. The variable resistance ballast may have a set of switches that selectively choose from a set of resistances (e.g., R, R/2, R/4 and R/8) based on the lamp operative voltage and power. The resistor material may be nickel-chromium resistance alloy wire. The heating of the resistor element may also serve in combination with a fan as thermal ballast for temperature control of the lamp.
The voltage across a gas discharge increases with distance and gas pressure. A mercury lamp contains low-pressure gas (typically argon) with a small amount of mercury. Current passing through the lamp increases its temperature evaporating the mercury causing its pressure to rise resulting in an increase in voltage across the lamp. The maximum lamp voltage occurs when all the mercury is in the vapor state and/or when the lamp is operating at its operating temperature (600-900° C.).
Typical power of about 70 W/cm is available from a mercury lamp within a few minutes after starting. At start-up a small pool of mercury is vaporized and heated. The lamp is a negative resistance coefficient device (resistance decreases with increasing operating power and temperature), requiring ballast to prevent increasing current from damaging the lamp. The decreased resistance is offset by ballast impedance that tends to limit current. As the lamp heats up during operation, internal gas pressure rises and a higher voltage is required to maintain the discharge. The resistive drop across the ballast supplies the required voltage until the required voltage cannot be supplied to maintain the discharge. At that point, the discharge is extinguished, the lamp cools, the gas pressure is reduced and the ballast is again effective once the lamp is started. An auxiliary high voltage electrode is used to restart the arc discharge.
Magnetic Ballast
Resistive Ballast
Lamp Voltage
A mercury lamp with a series resistance (R) connected to a voltage (Vin), has an initial voltage drop of approximately 15 volts and its current increases to (Vin−15)/R. This voltage drop is associated with the energy required to emit electrons from the metal electrodes into the gas. The voltage (V/cm) across the length of the tube depends on the amount of mercury evaporated and is Vlamp=m7/12d−3/2, where m is the amount of mercury (mg/cm) and d is the tube diameter (cm). The vapor pressure P of mercury (kPa) is a function of temperature T (K) and can be expressed as P=7.58×10−1 T+3.9×10−3 T2+4.8×10−6 T3. The pressure for an ideal gas is P=nRT/Vo, where n is the amount of evaporated mercury (moles), R is the gas constant (8.31 kPa L/K mol), and Vo is the gas volume in liters), with the mercury vapor pressure from above and substituting for the amount of mercury (moles), the lamp's steady state temperature (for a typical 7.5 cm lamp, d=2 cm) as a function of total lamp voltage (V) can be expressed as,
T(K)=6V+390, for V>15 (1)
The lamp's steady state envelope temperature can also be determined from power balance considerations as follows.
Lamp Temperature
To determine the lamp's operating temperature we look at the power loss from
Qinput=Qconvection+Qradiation-lamp+Qballast+Qradiation-arc (2)
where Qconvection and Qradiation-lamp are the power of the lamp lost by convection and radiation, respectively. The convective heat loss depends on the Nusselt number (Nu) as
Qconvection=Nuk(Ts−Tair)A/D (3)
where k is the thermal conductivity of air, Ts is the lamp's surface temperature (° C.), Tair is the surrounding air temperature (° C.), A is the lamp's surface area, and D is the lamp's diameter. For forced convection, the Nusselt number is
NUforced=0.3+0.62Red1/2Pr1/3{[1+(0.4/Pr)2/3]1/4}−1{[1+(Red/282200)5/8]4/5} (4)
where Red is the Reynold number and Pr is the Prandtl number. The power lost by radiation is
Qradiation={acute over (ε)}Aσ(Ts4−Tair4 (5)
where {acute over (ε)} is the emissivity of quartz (0.93), A is the surface area, and σ is the Stefan-Boltzmann constant.
Assuming Tair=300K and that the non-radiative loss heating the quartz lamp is nearly equal to ⅓ of the input power (ViI), by combining equations 2, 3, 4, and 5, and the ballast power (I2R) and substituting in equation 1, for a 7.5 cm lamp,
ViI=A(6V+112)+B((6V+390)4−3004)+I2R+2ViI/3 (6)
where A=Nu k A/D, B={acute over (ε)} A σ, and R is the series resistance. This can be solved numerically for voltage (V) as a function of current (I). The lamp's steady state V-I characteristics can now be determined by gas flow and external circuit parameters. By combining this with the load lines in
Ballast Design
With reference to
To limit ballast power to 50% of previous value the switches are switched as follows: % V2=(100+% V1)/2 where % V2 is the lamp's next switching voltage, and % V1 is the previous voltage. The voltage levels for 120 volts input are:
If the resistance R is known, PB=Vi2(100−%)2/(1002R), PL=Vi2%(100−%)/(1002R), and I=Vi(100−%)/(100R), where PL is lamp power in Watts and I is the current in Amperes. For a ballast resistance of 10Ω, the lamp powers for the examples above are 158, 709, 985, and 1119 Watts respectively. It is interesting to note that the lamp's overall electrical efficiency PL/(PL+PB) is equal to the lamp's voltage in percent and is for the above example equal to 89.1%.
To provide practical applications for resistive ballasted UV lamps, high power resistors are required, as are means to change their resistance as the lamp warms up. This can be accomplished with air-cooled resistance wire heating elements (e.g., made of Nichrome® or another nickel-chromium alloy) and semiconductor switching techniques to select the desired resistance value for the lamp temperature and voltage in order to keep the maximum ballast power below the limits described above.
A 2 kW magnetic ballast typically weighs about 30 kg and requires a volume of 8 dm3, while the corresponding 2 kW resistive ballast will weigh only about 0.5 kg and occupy only about 1/10 the volume of the magnetic ballast. This allows for handheld UV curing systems with self-contained ballast that can operate directly from a 120VAC, 12 amp power outlet via a plug and electrical cord. Such readily portable equipment is adaptable to curing small areas, such as countertops, bathtubs, patchwork, silkscreen inks for labeling on small parts, etc., as well as doing lab work for coatings development.
With reference to
The resistive ballast variable resistor in
UV Floor Curing Machine
Inherent in resistive ballasting is the use of short lamps (15 cm or less) that can be operated from either 120- or 220-volt outlets without transformers. Thus to cure large areas, the concept of rotating lamps has been devised. In these systems, two or more lamps rotate about a shaft suspended above the floor by centrally located wheels. Such systems have omni-directional motion and extremely high peak-power levels since multi-kilowatt power is applied across short lamps. Such systems can operate from 120- or 220-volt outlets.
With reference to
Lamp housings 93 contain UV lamps 92. The quantity and positioning of the UV lamps 92, lamp housings 93 and lamp arms 94 may all be varied. UV lamps 92 are connected to respective variable-resistance electric ballasts of the electric ballast system 82. The lamp housings 93 may be elongated structures that are parabolic in cross-section and constructed of sheet metal or plastic having reflective surfaces thereby enabling reflection of UV light emanating from UV lamps 92 as a downwardly directed linear beam onto a coating to be cured.
Rotary Lamp Variables
In operation, the UV lamps 92 may rotate to form a curing zone as the housing 83 is rolled across the floor. Since super-portability implies short lamps, we investigate the curing pattern of a two-lamp system rotating on a 50 cm diameter spindle that in turn can be rotated by the operator at some convenient rotation. The pattern of a point (r) on the lamp then follows the equation of a double trochoid:
x=vt−r SIN(2πθt)−R SIN(2πφt)
y=R−r COS(2πθt)−R COS(2πφt) (10)
where r=any point on the lamp, R=Machine radius, θ=the rotations per minute of the lamps, φ=the rotation per minute of the machine, v=forward speed of operator, and t=time in seconds. Although the spatial cure pattern for a single pass of the floor apparatus, a skilled operator will cover the entire floor similar to that with a rotary sanding machine.
UV Safety
Direct and reflected UV light from a 120 volt 1000 watt 7.5 cm handheld and 3 kW 50 cm diameter floor machine was measured with a WA meter in Watts/m2:
while for comparison the afternoon Sun measures 15.5 W/m2 in the UVA region. Accidental exposure at close range from any high power lamp can be dangerous. Direct exposure from within 1 meter of the handheld device can result in greater UV exposure than the afternoon sun. However, at normal operating distance, the exposure is ten and a thousand times less than the sun's exposure from the reflections of the hand-held and floor machine respectively, and should be treated appropriately with eye and skin protection, similar to that when working outdoors in the sun.
We have shown that it is feasible to operate mercury lamps with resistive ballasts making UV curing equipment significantly more portable and less costly than other UV curing technologies. Although resistive ballasts consume more power than the lamp during warm-up, it has been shown that UV systems with the variable-resistance ballasts of the present invention can achieve efficiencies equal to or greater than magnetic ballasts.
This application is a continuation-in-part of prior U.S. application Ser. No. 12/751,606, filed Mar. 31, 2010, in turn a continuation-in-part of U.S. application Ser. No. 12/478,970, filed Jun. 5, 2009, in turn a continuation-in-part of U.S. application Ser. No. 12/209,080, filed Sep. 11, 2008, now U.S. Pat. No. 7,731,379, which in turn is a continuation-in-part of U.S. application Ser. No. 12/112,753, filed Apr. 30, 2008.
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| Number | Date | Country | |
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| Parent | 12751606 | Mar 2010 | US |
| Child | 13020688 | US | |
| Parent | 12478970 | Jun 2009 | US |
| Child | 12751606 | US | |
| Parent | 12209080 | Sep 2008 | US |
| Child | 12478970 | US | |
| Parent | 12112753 | Apr 2008 | US |
| Child | 12209080 | US |
