COOLING SYSTEMS

Radar equipment, particularly the high-power transmitters, generate large amounts of heat. This heat must be dissipated to prevent damage to the equipment and to prevent erratic circuit operation. Most radar equipment rooms have high-capacity air-conditioning systems to control the ambient room temperature; however, equipment cabinets must have additional cooling to control the internal temperature. In the case of transmitters (and other high-voltage circuits), individual components may require cooling.

Cabinets that generate relatively small amounts of heat may only require a system of fans or blowers to maintain constant air circulation. In some cases the air is circulated through a liquid-cooled heat exchanger located inside the cabinet.

Most low-power amplifier tubes are air cooled; most high-power tubes, such as klystrons, crossed-field amplifiers, and magnetrons, are liquid cooled.

The main source of power and heat in a power amplifier package is the high-voltage power supply. Part of the power produced by the power amplifier is transmitted in the form of rf energy; the remainder of the power eventually converts to heat, and cooling is required to dissipate the heat.

Radars that use blowers for cooling will usually have an airflow sensing switch. If the blower fails, the switch will open and remove power from appropriate power supplies. Radars employing liquid cooling normally distribute the liquid into a large number of separate paths, because the flow requirements are quite dissimilar. Each of the various paths will have a low flow interlock. If one of the liquid cooling paths becomes restricted, the low flow interlock switch will open and remove power from the radar.

Liquid cooling systems also include pressure gauges and switches, temperature gauges, and overtemperature switches. Many systems have pressure or flow regulators. Some systems include audio and/or visual alarms that energize before damage actually occurs. In some cases this allows the problem to be corrected without turning off the equipment.

Figure 4-15 illustrates a typical transmitter cooling system showing the many protective devices.

Figure 4-15. - Typical transmitter cooling system.

Distilled water is one of the best mediums for cooling high-power components, and, in many cases, the only medium that may be used.

For a distilled-water-cooling system to operate satisfactorily, the temperature, quantity, purity, flow, and pressure of the water must be controlled. This control is provided by various valves, regulators, sensors, meters, and instruments that measure the necessary characteristics and provide the required regulation.

Liquid-cooling systems consist of a sea water or a chilled (fresh) water section that cools the distilled water circulating through the electronic equipment. The main components of cooling systems are piping, valves, regulators, heat exchangers, strainers, circulating pumps, expansion tanks, gages, and demineralizers. Other specialized components are sometimes necessary to monitor cooling water to the electronic equipment.

A typical liquid-cooling system is composed of a PRIMARY LOOP and a SECONDARY LOOP (figure 4-16). The primary loop provides the initial source of cooling water and the secondary loop transfers the heat load from the electronic equipment to the primary loop. The source of cooling water for the primary loop is either sea water from a sea water supply or chilled water from the ship's air-conditioning plant. The cooling water used in the secondary loop is distilled water. Ultrapure systems are maintained by a demineralizer and use double-distilled water obtained through the Navy Supply System.

Figure 4-16. - Liquid cooling system block diagram.

Additional information about liquid cooling systems can be found in Basic Liquid Cooling Systems for Shipboard Electronic Equipment Technician's Handbook, NAVSEA 0948-LP-122-8010.

Q.19 What type of cooling is used to control ambient room temperature?
Q.20 A typical liquid-cooling system is composed of what loops?
Q.21 What loop of a cooling system is often supplied by sea water?

SAFETY

Many safety and health hazards are involved with operating and maintaining high-power radars. These hazards result from high levels of rf radiation, X-ray emissions, the necessity of working aloft, and the generation of extremely high voltages.

Navy professionals are very safety conscious and, as a result, the number of accidents that occur on the job is small. Most of the safety precautions applicable to radar are published in radar technical manuals. Many of the safety regulations included in technical manuals are the result of actual experiences. Therefore, you should give them careful thought and strict observance.

RF RADIATION HAZARDS

Radar peak power may reach a million watts or more. Rf radiation hazards exist in the vicinity of radar transmitting antennas. These hazards are present not only in front of an antenna but also to the sides and sometimes even behind it because of spillover and reflection. At some frequencies, exposure to excessive levels of radiation will not produce a sufficient sensation of pain or discomfort to warn you of injury. If you suspect any injury, see your ship's doctor or corpsman. Be sure to acquaint yourself with the actual radiation hazard zones of the radars on your ship.

Personnel should observe the following precautions to ensure that persons are not exposed to harmful rf radiation:

Visual inspection of feedhorns, open ends of waveguides, and any other opening that emits rf energy should not be made unless the equipment is properly secured and tagged for that purpose. Operating and maintenance personnel should observe all rf radiation hazard signs posted in the operating area. All personnel should observe rf radiation hazard (radhaz) warning signs (figure 4-17) that point out the existence of rf radiation hazards in a specific location or area. (You may encounter other types of rf radiation hazard signs, depending on the situation.) Ensure that radiation hazard warning signs are available and posted. Ensure that those radar antennas that normally rotate are rotated continuously while radiating or are trained to a known safe bearing. Ensure that those antennas that do not normally rotate are pointed away from inhabited areas (ships, piers, and the like) while radiating. Dummy loads should be employed where applicable in transmitting equipment during testing or checkout.

Figure 4-17. - Sample of one type of radhaz sign.

X-RAY EMISSIONS

X rays may be produced by the high-voltage electronic equipment in radars. X rays can penetrate human tissue and cause damage of a temporary or permanent nature. Unless the dosage is extremely high, no ill effects will be noticeable for days, weeks, or even years after the exposure.

The sources of these X rays are usually confined to magnetrons, klystrons, and cathode-ray tubes. Personnel should not linger near any of these types of equipments when the equipment covers have been removed. Klystrons, magnetrons, rectifiers, or other tubes that employ an excitation of 15,000 volts or more may emit X rays out to a few feet; thus, unshielded personnel standing or working close to the tubes will be endangered.

When performing maintenance on X-ray emitting devices, you should take the following precautions:

Observe all warning signs (figure 4-18) on the equipment and all written precautions in the equipment technical manuals.

Figure 4-18. - X-ray caution label.

Unless called for in the technical manual, do not bypass interlocks to permit the servicing of operating equipment with the X-ray shield removed. Be sure to replace all protective X-ray shielding when servicing is complete.