In an article in an earlier issue, I introduced basic concepts of liquid cooling and considerations to keep in mind when shopping for an FM transmitter. 

The topic brings up more questions than answers, so let’s try to clarify some of the issues.

Liquid cooling circulates “anti-freeze” through various elements of your transmitter. Generally, this includes the power amplifier modules, possibly the power supplies and of course the pumps, which may be housed inside the transmitter or in a separate rack nearby. 

When a liquid-cooled component is removed, the valves of that component, both in and out, must be shut off. In some cases, you’ll bypass the component by turning valves manually, in other cases, the valves are automatic. “Plug-and-play” takes on a more complex meaning when liquid is a factor. 

Case study

This is an example of how to bypass a defective pump for replacement.

Fig. 1 shows a panel on a Rohde & Schwarz model (the two pumps are visible at the bottom of the picture). It provides a parallel pump scenario that could be found in many types of equipment that use liquid for cooling. 

There are two valves visible, VH1 and VH3, set to Normal Mode. The diagram, in closeup in Fig. 2, shows how to turn the valves for maintenance modes for Pump 2 or Pump 1. Setting these allows you to bypass the malfunctioning pump and replace it while losing only a small amount of fluid. 

Once the liquid is no longer going through the defective pump, you can replace the pump. Fig. 3 provides a view of the pumps; you can also see VH2 below the pumps.

Luckily, you don’t need to replace the heavy black iron of the manifold on the back of the pumps. You can remove the pump from the front of the manifold with four silver Allen screws. 

The tougher part involves the electrical and control connections; you need to take apart the front readout cover and disassemble the front of the pump. Caution: One of the small connectors has high voltage, so you must trace both cables to the control board and disconnect before you unplug anything. 

Other considerations

Maintaining a liquid-cooled system adds to your upkeep routine, but the main challenge will be completing the installation and setting liquid levels correctly in the transmitter. 

The heat exchangers must be installed adjacent to the transmitter on the outside of the building. This involves cutting holes in the building for the liquid plumbing and the wire harnesses that connect the heat exchanger, fan monitoring and controls to the system. 

Then your custom “plumbing” also may involve cutting large copper pipe or hoses, assembling connectors, routing and supporting the piping. 

Once the system passes a pressurization test, liquid coolant is pumped into the system. Typical steps include priming the filler pump manually and filling the system with a hose and buckets of coolant provided by the transmitter manufacturer. (This task is almost impossible without several people.) 

Typically, bleeding out the air involves opening valves at the highest point of the plumbing. Those will need to stay open for hours or days to finish this task, though the transmitter may be allowed to run in some configuration. 

The transmitter control system should be providing the data necessary to determine if the liquid-cooling system is coming up to proper pressure and the pumps and heat exchanger fans are operating correctly. 

Other than these liquid aspects, your procedure for installing a new transmitter is much the same as with an air-cooled transmitter, without the exhaust fans of course.

A few factors I look for when recommending transmitters:

1. In any transmitter, power amplifier modules are going to fail. Typically, these cannot be repaired in the field. Ask up front about replacements, repair policies and warranties. Investigate the difficulty in changing them out and shipping them. 
2. Power supplies: Are they custom-made for this transmitter, or generic and available “off the shelf” from various suppliers?
3. The liquid used. Is it available only from the transmitter company, or can it be made on site by combining automotive antifreeze and water?

Every situation is different. But speaking generally, liquid cooling will reduce heat loading in your transmitter building, saving you money that you might spend to buy, run and maintain larger HVAC systems to support air-cooled transmitters. Removing heat from components through the direct contact of the liquid is more efficient than air cooling. But the complexity of liquid cooling will involve more upfront costs. 

It all comes back to return on the investment. Perhaps the biggest question for a station owner is how long they plan to own the station. Fluid cooling will almost always save money over the long run. Transmitter companies can provide an analysis to help the station owner choose between liquid- and air-cooled models and estimate how long it will take to repay the additional cost of fluid cooling through reduced electricity usage.

Read a 2019 article by Don Backus of Rohde & Schwarz with more about maintaining liquid-cooled systems.

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