THERMAL CONSIDERATIONS IN SHELTER DESIGN

One of the most significant factors affecting TWT and Klystron tube life is temperature. Increases in operating temperature will create a proportional decrease in tube life. Most systems produced by MCL are designed with cooling systems to ensure reliability and long life at temperatures up to 52 deg C and altitudes up to 10,000 ft. Good shelter duct design can also increase that reliability and life. The following guidelines offer some tips for good shelter design.

External Ducting

Any back pressure or resistance in the ducting will decrease the airflow. Most HPAs are designed with some amount of additional back pressure capability. This capability varies by HPA and can be found in the Operations Manual. Design external ducting to minimize back pressure. Use the largest cross section ducting practical. Avoid long runs, sharp bends and rapid expansions or contractions of the ducting. Use helper fans where possible.

Shelters With Multiple HPAs

If several HPAs are connected to common ducts, the total back pressure from all must not exceed the maximum recommended back pressure for a single HPA. The use of a helper blower located in the duct is strongly recommended. The blower must be designed to handle the air volume and temperature.

If several HPAs are operated on common ducts without auxiliary blowers, the intake duct will operate at a lower than ambient pressure because air is being drawn out of it and the exhaust duct will operate at a higher than ambient pressure because air is being pushed into it. If one of the HPAs is turned off, the air flow will begin to cycle backwards from the exhaust duct through the HPA to the intake duct. This has a potential for causing two problems. First, the exhaust air will be heating the HPA. Parts not normally exposed to tube exhaust temperatures could be damaged. Second, the exhaust from the operating HPAs will cycle through the turned off HPA and heat the intake air supply. This could start a runaway temperature condition. This information is given as a general warning and design guideline. These effects have not been tested by MCL.

High Humidity Conditions

In areas of high humidity, a danger of condensation build-up exists when unconditioned air from the outside is delivered to the cold HPA. The HPA could be cold because it has been off or because the shelter air conditioning system is cooling surfaces exposed to outside air. Condensation checks should be made whenever a HPA is started or when shelter temperatures are more than 10 deg F (5.5 deg C) cooler than the air used to cool the HPA and high humidity conditions exist. Operating HPAs in standby mode in air conditioned shelters for long periods of time often causes this problem.

Air Filters

Develop a regular maintenance plan for the inspection and cleaning of air filters. Air filters should be maintained at least once a month. If the filters are found to be excessively dirty, the frequency of the maintenance period should be increased. The frequency may also need to be increased during dry periods or when there is construction nearby. In highly corrosive environments the filter should be replaced at regular intervals.

Phase Order

Check the phase order of three phase blowers. Improper phase order will cause blowers to rotate backwards. Some centrifugal blowers will still blow air the correct direction but at a highly reduced rate. Rotation of blowers can be visually inspected during turn on and turn off.

Doors and Covers

Do not operate systems for extended periods of time or at high ambient temperature conditions with the HPA covers off or doors open. In many cases these are an integral part of the cooling design. Air will not flow correctly and some times will not be cooled adequately. Air filtering will also be compromised.

Helpful Conversions

As you setup your ventilation system, the following formulas might prove to be helpful:

Estimating Barometric Pressure

Calculating Air Density

Converting lbs/hr to CFM (cubic feet per minute)

Setting up and maintaining an appropriate HPA ventilation system is important to the operating efficiency and life of your amplifiers. Since every installation is a little different, don't hesitate to call the MCL Service Department to discuss the specifics of your installation. We'll be happy to share our experience based on hundreds of HPA sites.

For more information, please contact us.

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