During the construction of compact power amplifier (PA) for a radio station alternatives abdouni lamps no. This is confirmed by international practice, so as lamps are used in most modern branded amplifiers.
One of the important structural elements of the amplifier can be called a system cooling of the lamp. Information on the design of such systems in the literature virtually no, and that's probably the biggest "white spot" in the "utilitylibrary". Meanwhile, this information is important because the layout of the MIND depends on the design cooling system, and if a wrong decision will require time-consuming rework. The cooling system should be done right the first time.
The article presents practical study design parameters of air cooling systems generator lamps.
The choice of evaluation parameters for the testing of cooling systems and measurement technique
The passport is a powerful generator of lamps, the manufacturer specifies the conditions cooling and maximum temperature of structural elements [1]. Therefore, the first and main evaluation parameter when comparing different systems blowing working radio tubes adopted the maximum temperature of the anode heat sink \max-
Cooling of the lamp is dependent on the supply (flow) of air by fan [1]. So for the most efficient use of the air flow of the air tract the amplifier shall have a minimum aerodynamic resistance (hereinafter resistance). It is, in General, due to the location fan shape radio tubes, its panel and configuration of the duct.
Moving in the duct, the flow is characterized by velocity v, m/s, and flow V=v-s, m3/s, where s is the cross sectional area of the duct at the measurement location speed, m2 [2]. All resistance in the path of the air flow causes the reduction of speed, and consequently loss of feed.
These values can be used to assess the resistance of the air tract. Therefore, the second evaluation parameter in comparative tests of the systems cooling is adopted, the reduction of feed AV, expressed in % AV = [(V b is-V)/V b is]-100%,
where V is the supply fan in the system air flow, m3/h;
V b is - flow fan in the base case with which it is compared, m3/h.
For example, the supply fan installed in the empty duct, V b is= 120 m3/h. When placed in the duct panel with radio tube supply decreased to 53 m3/h. Reduction of supply due to their resistance will be
AV= [(120-53)/120]-100 % = 56 %.
The second auxiliary parameter can be used when comparing systems cooling without a working radio tubes.
For experiments we tested the system blow-lamp GU-84B, consisting of standard panels, air ducts with an internal diameter of 112 mm and a fan.
It allowed to test different cooling systems and their individual elements. During the test, a tube was employed as the heat source, i.e., applied to all the anode power RA turned into heat.
Air supply was determined by a vane anemometer (intended for testing ventilation systems) [2] located directly behind the duct.
The temperature was measured with a digital multimeter M with thermocouple. Error the measurement was ±3° at t < 150 °C and ±3% at t > 150 °C. the Temperature was determined after ten minutes of lamp operation in the measuring mode.
Cooling system with axial fan
Almost four variants blowing tubes: lateral, axial supply, axial and axial exhaust dual-fan supply and exhaust. Best of them was determined almost cooling efficiency.
Test was applied axial metal fan TYP 4658N with the impeller diameter of 110 mm and n = 2200 rpm Supply fan in an empty the duct - 120 m3/h.
In a side sweep (Fig. 1) the cooling air passes only through a portion of the ribs the lamp heat sink and the cooling surface is reduced 9...21 times (table. 1). To improve the cooling of the can, increasing the air speed, but it will increase size and fan noise. The ineffectiveness of the scheme is obvious. The manufacturer also does not recommend the use of a lateral blowout for lamps designed for axial the passage of air [1 ].
The test results of the extract (Fig. 2) and inlet (Fig. 3) ventilation systems presented in table. 2.
The measurements showed that the flow of exhaust fan in the system (53 m3/h) in 2.4 times more than in the inlet (22 m3/h). If you make a comparison of the heat sink temperature which can be measured more accurately, then tAmax = 130 °C is reached in the supply circuit when RA = 240 W, and in the exhaust circuit tAmax = 126 °C, With RA = 460 watts. Therefore, exhaust fan removes about two times more heat than the supply.
For a man accustomed to deal with electric circuits, this result it may seem unexpected. Indeed, any resistor causes the same the voltage drop regardless of which side of the power source he is. The laws of motion of air differ from Ohm's law, and aerodynamic the resistance of the lamp to the panel in this case depends on the location fan. The result is explained as follows.
The flow of air from the axial fan, ramjet, and swirling (twisted, as the threads in the wit rope), and he enters into the annular gap panel not perpendicular, but at an angle (Fig. 3). Swirling the air at the entrance to the panel behaves like a stone thrown into the water at the angle; repeatedly Bouncing off it before diving. Therefore, 82% of the supply fan is lost to friction between the individual layers of the flow. This degrades the heat dissipation.
When the exhaust fan is under the influence of low pressure passes through the lamp in-line flow, so the reduction in flow is much less. In this if it is mainly caused by a frontal collision with the cathode.
Insufficient air supply can be increased in two ways: to apply more powerful fan, or install a second fan disposed coaxially with the first. For determine the best way were tested dual-fan system blowing.
It is established that the efficiency of feed dual fans depends on the distance between them. At a distance of 30 mm increase in the feed was 5 %. The reason, obviously, that a swirling air flow from the first fan enters the blades of the second under non-optimal angle, not captured by these blades, and is reflected from them. With increasing distance up to 100 mm feed increases by 30 %, as the airflow from the first fan gets axial direction, and more successfully captured by the blades of the second fan. Obviously, as the distance increases the efficiency of the second the fan will continue to grow. But long duct will increase the dimensions and make it more difficult layout. Therefore, the use of twin fans unnecessarily.
Joint work of two sources (transducers) energy always was challenging and demanded the application of special technical solutions. Obviously, for a concerted effort by fans should pick up the distance between them, the shape and mutual arrangement of the blades, as well as set "rectifying" the airflow plate. In any case, this task is already out beyond the "pricelistname".
Axial dual-fan forced-air blowing scheme shown in Fig. 4.
On the measurement results are shown in table. 3, it is seen that after joining the scheme of the second exhaust, supply air fan supply air only increased 20 %, a tAmax decreased by 8 %. Therefore, the application of the second, the supply fan is ineffective. The causes of this phenomenon have already been considered above.
According to test results of different variants of ventilation with axial fans we can draw the following conclusions:
1. Optimal is the exhaust system cooling with a single fan, provide the necessary air supply.
2. The application of the second fan to increase the flow in any unreasonably the cooling system.
Substantiation of the design parameters of the exhaust cooling system with axial fan
When RA = 460 watt and the gap In between the heat lamp and duct equal to 7 mm, the distance between the fan and the anode heat sink was set equal to 50, 80, 115, 150 and 210 mm. the measurement Results are shown in the graph (Fig. 5).
With decreasing distance And up to 50 mm heat lamp is in the zone of turbulence in front of a fan and tAmax is increased by 10% due to the deterioration of cooling. When significant removal of cooling fan is also deteriorating because of increasing the loss of kinetic energy of air friction on the long wall of the duct. Best cooling conditions are provided when And equal to 1,0...1,2 diameter fan.
The air temperature in front of a fan as the distance from the anode decreases with 97 to 49 °C due to cooling through the wall of the duct. For better heat they must have a minimum thickness.
The temperature of the blades is smaller than that included in the fan airflow. It due to the fact that the hot air coming from the fan, intensely mixed with outer, cools quickly and cools itself by external parties of the fan blades. For the same reason, with the decrease And the temperature of the blades grows more slowly than the temperature of the hot air in front of a fan.
The measurement results are shown in table. 4 show the dependence from tAmax the gap In at RA = 770 W and A = 115 mm.
If gap = 0 side surface of the heat sink is not involved in the heat transfer and the maximum anode temperature. When B = 7 mm tAmax decreased by 15 °C, as cooling began to engage the side surface of the heat sink. With the increase gap In to 17 mm tAmax decreased by 5 °C. When increasing the gap increases air velocity at the outer side of the heat sink, so the improvement cooling is possible, but the difference with previous experience does not exceed the error measurements. Therefore, for effective cooling of the outer surface heat sink lamp enough clearance 5... 10 mm.
Based on the above results was made and tested exhaust system cooling for the lamp GU-84B (Fig. 6).
Measurements have shown that tAmax is achieved when RA = 770 watts. The temperature of the blades the fan is then equal to 73 °C, so all metal fan with maximum power will provide greater reliability.
Do fans of plastic parts maximum operating temperature up to 60 °C [3,4].
With increasing RA from 0 to 770 W tAmax increased from 36 to 207 °C, and the cathode - from 120 to 145 °C. Therefore, cooling of the cathode of the lamp, even when its maximum heat mode, just an exhaust fan.
In Fig. 7 shows the dependence tAmax the time of heating at RA = 770 watts and cooling when RA = 0. The full warm-up bulb after submitting all voltages - 10 min cooling to 36 °C - Schedule 11 min. cooling of the anode allows you to calculate the temperature of the amendment to measure the temperature of the anode is not in transmission mode, and after a period of time required for shutdown threat stress.
The dependence in Fig. 7 explains why even with inefficient cooling system the operational amplifiers in CW and SSB modes.
When the daily work time transmission does not exceed, as a rule, 1...2 min and the lamp just does not have time to warm up, and while receiving it is quickly cooled. Therefore, the intensity blowing in CW and SSB modes can be several times lower than in continuous radiation.
Cooling system with centrifugal fan
Tested three systems with centrifugal blower fan: blower coaxially with flow (Fig. 8), exhaust (Fig. 9); the supply side flow (Fig. 10).
For the tests applied centrifugal fan with impeller 30 mm wide and with a diameter of 92 mm, which was rotated by an electric motor KD-3,5 AU n =1400 R/min. Supply fan in an empty duct - 90 m3/h.
The test results showed (table. 5) inlet centrifugal fan with coaxial flow is most effective. Its uniflow airflow and has high speed v than that of the axial fan. With the same air supply it the kinetic energy is much higher, because it is proportional to v2. Rapid steam better airflow overcomes the resistance the air tract, and contacting with the lamp, provides a large heat transfer. The fan is working in the best conditions. Here the supply of the cold air, therefore, you can use easy plastic impeller, to reduce the load on the bearings and extend their life. The motor is shielded from RF radiation by the walls of the inlet compartment. The use of an electric motor with bearings made of porous bronze allowed to minimize the noise level.
The inefficiency of the ventilation supply ventilation system with lateral flow (Fig. 10) visible without tests, as the air, hitting a wall, it loses much of its kinetic energy and only then, the rebound, goes to the lamp. The measurements are performed, to compare the quantitative performance of this and other systems. Results tests (tab. 6) showed that the lowest losses are achieved with minimal the size of the input compartment, i.e., when it is actually a continuation duct with a lateral outlet. In this case, the flow, compared to coaxial flow (Fig. 8, tab. 6), less than 2.8 times, a tA max above 70° C or 1.7 times.
The advantage of the system with side stream to simplify installation of AHU. It can be placed on either side of the lamp and to maintain a small height MIND. Downside - the worst of the heat sink due to the significant loss of supply fan (80 ...85 %) when turning the air flow.
This system is used in business INTELLIGENCE. It is efficient in the application small lamp (GU-74B, PG-B) that require small air consumption [5].
The effect of anode attachment for cooling the lamp
No significant difference in the cooling of the lamp with the "anode fixture" and without him was not. The repeated comparison of tA max lamp, mounted in a proprietary anode ring and without this attachment, the difference was within the measurement error (when other conditions being equal).
Fastening to the anode ring is necessary for reliable fixation of the lamp. But if the user panel was without anode ring too, it can apply. Manual allows for mounting the lamp in the panel to focus on the second ring mesh with a clamp lamp from the anode [1]. To implement this attachment instead of the missing signature of the anode ring install the duct in which the insulators are placed for emphasis pressing the lamp from the anode. This method is especially convenient when using exhaust diagram-cooling with axial fan.
The definition of the supply fan in SSB and CW modes
All the above measurements were obtained after 10 minutes of work the lamp that corresponds to the modeling of continuous radiation. For SSB and CW average heat dissipation at the anode will be much less. In this case fan speed (and hence noise) can be significantly reduced.
Depending on the duration of transmitting the ratio of the time RX/TX, view radiation, the quiescent current and the peak factor SSB signal average power dissipated at the anode, can be reduced in several times. For example, when working CW, given pause, the average power is 60. ..70% of the mode settings. During receiving the lamp cools quickly (see Fig. 7). If we take the ratio of RX/TX 1:1 and transfer time (1 ...2 min), the time of admission may be counted in the calculation average heat dissipation for the lamp. In CW it will be about 3 times less than continuous radiation.
Using the found coefficient and efficiency of the amplifier is easy to calculate the output the power at which the tested system can cool the lamp. But it a rough calculation based on certain assumptions.
Accurate calculations of heat dissipation at the anode in CW and SSB modes complicated and unnecessary. More than just to determine the necessary flow (rpm) of the fan according to the temperature anode in real operation conditions.
For example, in the cooling system of the MIND for GU-43B [6] the speed of the fans were reduced so that when working SSB thermal protection lamp worked through 15 minutes. This is more than enough for any practical work. As a result adjust the fan noise was less than the noise from the speaker when the average volume.
Well-made cooling system will provide the operator with comfortable on the radio speaker, and a tube will completely fulfill the planned resource.
The reduction of noise during operation of the cooling system
The fan was escorted two main sources of sound - the motor and fan blades. Moving in the duct flow creates very little noise.
The main source of sound in the motor is the bearings. Therefore, you should to use special low noise bearings of porous bronze. In the collector engine noise occurs when the friction of the brushes on the collector.
We should pay particular attention to the way the motor mounts the centrifugal fan. The sound of the engine attached to the body "snails", is enhanced by sonic resonance. It should therefore be attached to the body MIND. For massive chassis the motor is not strong exciter, and the resonant frequency of the body due to its size and weight is much lower disturbing frequency. To reduce the vibration of the engine it is to be low voltage These measures plus vibration allowed to completely from the sound resonance of the motor.
Strong sound is created when the rotation of the impeller. Therefore, the next task is decrease the speed of the meeting of the blades with air. This problem can be solved for through the use of centrifugal fan. The sound of an axial fan, installed at the outlet of the cooling system, freely distributed in the surrounding space. A centrifugal fan zone operation of the impeller, where the formation of sound waves, separated from operator double acoustic screen. The first is the fan housing ("snail"), the second wall of the housing MIND. In addition, a centrifugal fan the air accelerates when repeatedly exposed to the working blades wheels. Each blade gradually increases the flow, so the speed of its the collision with the air and the noise is less than the axial fan. With decreasing the velocity of impact sound frequency decreases and is shifted to the minimum the sensitivity of our ear.
When using an axial fan, the noise is reduced by the optimization of the system blowing. The application of the exhaust cooling system with optimal parameters, compared with the supply, will reduce the supply fan and the speed of the blades 2.5...3 times. Some noise reduction can be obtained when placing the fan on the rear panel of the amplifier [6]. In this case, for the operator housing amp is an acoustic screen.
The next way is to apply axial fan possibly larger diameter, but decrease the speed of rotation of the impeller. (The speed of passage of air through the lamp remains unchanged).
Fully audible noise when the blower is not to eliminate, but well-made MIND they are extremely minor. The above methods will enable you to achieve a good results with any lamps.
Conclusions on test results
1. For cooling the lamp is most effective with one fan sufficient supply. The use of dual-fan system unnecessarily.
2. Due to the nature in the organization of the air flow axial fan creates a once-through flow and more efficiently in the exhaust system cooling and centrifugal fan - in supply and cooling system.
3. Test results cooling systems are two of the most efficient designs.
By the set of all parameters is the best supply cooling system coaxial flow from the centrifugal fan. It provides maximum the efficiency of AHU, minimum noise and reliable operation the fan, as it takes the cold air. Disadvantages - complexity of installation in the input compartment, the low prevalence of necessary fans and the motors on the market of components and high cost.
The second option is the exhaust system cooling with axial fan. It disadvantages - increased noise level and heat fan. And the advantage is the minimum dimensions and multiple to simplify the installation. In addition, axial fans significantly dusable than centrifugal installation, and in the market accessories you can easily find the required sizes.
Justified both the cooling system, the Final choice will depend on availability components, the layout of the amplifier and the author's opinions of the design.
To protect the lamp from overheating
Metal and ceramics have different coefficient of thermal expansion. In excess of maximum permissible bulb temperature, mechanical stress, caused by extension, can exceed the tensile strength of ceramics. Caused by this cracks will lead to a rapid loss of vacuum.
Protection of lamp failure of AHU in professional MIND is made using the sensor of the air flow. In the absence of airflow trigger it aerocontact and automatic de-energizes the lamp. As often aerocontact just used a reed switch, and the actuation is achieved by a miniature magnet mounted on a movable plate, which rotates the air stream.
This protection has two drawbacks: it does not protect the lamp from overheating when the detuning of the P-loop and blowing small lamps air flow will be insufficient for triggering mechanical sensor.
If you fail to achieve reliable triggering aerocontact, you can apply relay protection scheme (Fig. 11).
At break in the circuit of the motor control relay K1 is de-energized, contacts K1.1 are closed, and include Executive relay K2, contacts K2.1 turns the lamp off. Actuation of protection is indicated by an led VD2. After eliminate the breakage of the current in the circuit to the electric motor causes actuation K1, contacts K1.1 and opens the protection circuit goes into the initial state. In excess of the current in the motor circuit fuses fuse FU1 and then the circuit the protection is triggered, as in the cliff.
Emergency stop of the fan may be due to his refusal or the power outage.
In this case, the universal means of protection against overheating is the presence of a separate emergency fan, which is located in the same housing, with batteries. When you stop a regular fan, the operator sets an emergency the fan on the amplifier above the duct and cools the lamp within 5 minutes, as this requires manual [1 ].
When excess heat generation at the anode (for example, because of the detuning P-loop) nominal air supply will not be enough. To protect the lamp in this case should continuously monitor its maximum temperature. Point the greatest heat is in the upper inner part of the anode radiator. With the constant mode of operation of AHU air temperature for the anode and the anode temperature is well-defined dependencies (see Fig. 6). Therefore, easier to control the temperature of the anode, and the temperature air behind the anode.
After installation of the cooling system will need to experiment to get the data the temperature field at the anode. Then the temperature sensor, the temperature of activation which could be 70... 120 °C, is placed in the corresponding point duct.
With the closure of the contacts of the temperature sensor SA2 relay contacts K2 and K2.1 disconnect the lamp (Fig. 11). Contacts SA2 after activation remain closed for some time, yet removes heat from the anode. About operation protection indicated by an led VD2. After cooling, the lamp protection circuit itself returns to its original state.
The placement of the cooling system in the housing of the amplifier
Amplifiers traditionally used horizontal enclosure type "DESK TOP". On this reason, historically and rationally for old glass lamps layout "automatically" transferred to Abdoulie lamp. To save the traditional design and ease of installation of AHU used parallel compact GU-74B (or GU-B) and supply diagram blowing with a side stream. But due to large losses when turning the air this scheme not attractive to powerful (see tab. 6).
Amplifier given power always easier and cheaper to make one big the lamp is on. Therefore, the layout of the power amplifier must ensure installation of the most the effective cooling system.
To meet this requirement, it is necessary to abandon the traditional horizontal hull "DESK TOP", and use a vertical enclosure type "MINI-TOWER". Successfully is the most effective cooling system with coaxial flow the centrifugal fan or the most simple exhaust cooling system axial fan (Fig. 12).
Literature
Author: W. Karowski (RA1WT), Velikiye Luki