Currently have a tube final FM transmitter. TPO 3,600 watts. Looking at A/C cooling for the transmitter room, but may soon be upgrading to a solid state transmitter. Is there a huge difference in heat generated between a solid state transmitter and a tube final, for a TPO of 3,600 watts?
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A/C requirements for solid state vs. tube based
- Thread starter Brian Bowers
- Start date
That depends on the transmitters. Assuming the TPO is 3,600 watts (as mentioned), your transmitter is probably rated at 5,000 watts. For both solid state and tube transmitters, the final amplifier is not directly linear when it comes to heat dissipation vs RF power output. A tube will always generate a certain amount of heat with the filaments lit with zero output. A solid state transmitter generates much less heat by comparison, but does usually have sweet spots of output verses thermal efficiency. In some cases this effect is based on the design, type of output devices (MOSFET's vs. LDMOS), etc. For example, some solid state amplifiers will generate more waste heat operating at 50% RF output than 100% output.
What brand and model of transmitter are we talking about here?
What brand and model of transmitter are we talking about here?
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Harris FM 5K with a tube IPA. Still comparing the SS options. Any recommendations for A/C tonnage for this application? I've always been told that it's not good to oversize A/C capability, to prevent frequent compressor cycling.
I won't get into the comparison part here (already well covered), though I think I might look for a solid state IPA though. As for oversizing the AC, I think the new split-type inverter units largely take care of that. They ramp up and down, like if you had an adjustable rheostat on the compressor, based on the immediate needs of the system.
I did have this problem at a transmitter site in Louisiana. The old AC was so fast, it would shut off after basically cooling the air enough to satisfy the thermostat. After a short time, the stat would again call for cooling, but the gas pressures wouldn't have had enough time to drop and the compressor wouldn't start, thereby eventually blowing a breaker. I put in one of those Mitsubishi mini-splits, and that fixed the problem. There's plenty of capacity, when needed. After the building cools enough, the compressor idles down to where it either shuts off or runs just fast enough to maintain the set temperature.
And you plan on dumping the exhaust air into the room? The FM 5K is an old, rather inefficient transmitter, especially with a tube IPA. Converting this transmitter to a solid state IPA is possible, but probably a total PITA and wouldn't bring you much of a heat savings.
According to the original manual http://gates-harris-history.com/manuals/fm/8881761012.pdf "2-9 Cooling Air Requirements" the transmitter needs a minimum air flow of 400CFM with a maximum intake temperature of 120 degrees F.
Assuming your transmitter building is 26' X 26', minimal roof/wall insulation, and containing heat-generating gear other than the transmitter, I'd estimate a minimum of 34,000 Btu's and 1,144.00 CFM of cooling would be required. 34,000 Btu would be around a minimum 3 tons of air conditioning. (2.86 to be exact)
Depending on the solid state transmitter, the number of Btu's required for adequate cooling would probably be about half. Caveat is: My estimate doesn't take into account for worst-case outdoor ambient temperatures, other equipment or uses in the building, humidity, etc.
For example, recently I did a similar calculation for a building in Baghdad Iraq. I ended up using a typical daytime summer month ambient air temperature of 130F. This drove up the air conditioning load by almost 40%. In other words, there is a lot to consider when designing cooling for a TX site, not just the transmitter.
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Yes, dumping the exhaust air into the room. If the FM 5K will be kept for backup, I will definitely ponder a SS IPA conversion at a later date. The transmitter building is actually a room connected to the rest of the studio/office building. It's about 20' X 15'. Getting back to my question, if this room had a 5,000 watt solid state transmitter running at 3600 watts TPO, 3 ton would still be sufficient, but not overkill? How about if we purchased a Nautel V5? Here's the cooling requirements on that model:
Cooling Air Requirements
833m3/hr (490 cfm)
Cooling Air Requirements
833m3/hr (490 cfm)
You should be okay with 3 tons of AC for a 5kw SS Nautel if the room is well insulated and your not in an excessively hot area. I would recommend you have two AC units 3 tons each as a backup. Your best bet is just to have a chat with the manufacturer and ask for a AC ton recommendation, then get two redundant units.
OK, thanks for the help everyone. I too was thinking of calling the manufacturers (Nautel and GatesAir) for their opinion. In this particular situation, I doubt that I'll get a backup A/C unit for the main A/C unit (even though that would be wonderful). I'm not sure that it's needed in the Midwest. For decades, the room has been simply moving air in and out using a large filtered air intake on the wall (with no electric damper type system), with two thermostatically controlled fans to exhaust, creating a negative pressure in the room. It has done the job for all these years. That will be kept for an emergency backup cooling system, which will then be connected to the transmitter remote control, to alarm, if it turns on. Thermostats for the fans will of course be set much higher when the A/C goes in. I use a system just like it at another facility (one A/C unit). It seems to work pretty well. I have seen the exact system in the past at a previous company, but they went a step further, and turned it into a closed loop system, utilizing an electric damper on the intake vent, only opening when the backup fan system was needed. Somewhere down the line, I may integrate the electric dampers into my current filtered intake vents.
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