Metro discusses hot cars issue

As summer heated up this year, many riders rediscovered an old frustration: Metro cars with malfunctioning air conditioning. On sweltering days, these cars can be even hotter than the outside.

After seeing so much angst online from one of Metro’s most frustrated customers, @fixwmata on twitter, I asked Metro spokesperson Dan Stessel for some help with the hot car situation.

In order to understand why WMATA has so much trouble with their railcar air conditioning units, it might be helpful to discuss how they work.

Like most household and commercial air conditioning, Metro uses a technology called vapor compression, which has been in use for over a hundred years. It works by pumping a refrigerant to a high pressure and temperature. Then, in a heat exchanger called a condenser, air pushed by a fan removes heat from the hot refrigerant, causing the hot gaseous refrigerant to condense and become a liquid.

This high-pressure liquid is forced through a pressure reducing valve, making it cold. The cold fluid is sent through another heat exchanger called an evaporator where heat from the air is transferred to the cold refrigerant, causing it to boil. The air becomes cold and is sent to the space to be cooled, and the refrigerant warms up, boils and becomes a gas again. The cycle repeats when the gas is pumped to a high pressure again.

Metro uses two different refrigerants. The agency uses R-22 for the older cars and R-407C for the 5000 and 6000 series cars. Each car has two air conditioning loops, one for the front and one for the rear.  An evaporator and fan are at each end of the car, and two compressor-condenser assemblies are located in the middle of the car. Each air conditioner provides up to 8 tons of cooling, or about 2–3 times the amount that a typical house might have.

A number of things could go wrong and cause the air conditioner cycle to stop.  Electrical controllers for the pump or fans could overheat and cause their circuit breakers to open, protecting the system from fire, but also turning off the flow of refrigerant or cold air.  The refrigerant loop could spring a leak, and the pump would shut down because there is too little refrigerant to move around.  Since the condenser is located under the rail car, the heat exchangers can get clogged with leaves and debris, and which means the pressure downstream of the pump would become too high for the pump to work against.  Also, the pump could break or sieze up.

Metro checks the air conditioning system’s health during the daily safety test. They do not put cars out for passenger service with non-functioning AC, but sometimes a car is put out when the AC system is not operating at 100% of its rated capacity. Metro depends on these daily safety tests or employee reports to find cars that are not working properly.

Metro said that having customers report hot cars helps bring road mechanics to the scene. Sometimes the fix is as easy as resetting a circuit breaker, but more often it requires isolating the car and then removing it from service at the end of the line so it can be repaired.

Metro’s AC failure increases dramatically when it is hotter than 93 degrees out. For example, there were 9 total failures on July 27 with a peak of 93 degrees, but on July 29 there were 63 failures with a peak of 104 degrees. Some of the equipment would be capable of cooling a car with the lower heat load, but extreme temperatures cause more failures and lower performance. Having cars wait outdoors at the end of the line with the doors open is particularly bad for the system.

The hot cars reported after August 1 on fixwmata.com were reviewed, and all of them have been identified as needing some corrective action.  Six of those cars are still out of service for AC repairs.  Sometimes, there are repeat failures, with 50 repeat failures out of 1104 cars.

Metro said that they found a fix for the 5000 series cars and have begun buying materials to fix the issue.  A software improvement will also help the 5000 series AC systems once they are refurbished.

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About perkinsms

I'm an engineer and father interested in transit, parking and economics.
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One Response to Metro discusses hot cars issue

  1. Hai says:

    Hi Perkinsms, I am Hai Huang, a Georgetown graduate student. We are currently doing a mobile app project about DC Metro, I googled and found that it is precisely what you are interested in. May we interview you through email or phone? Thank you very much and I am looking forward to hearing from you.

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