One day long long ago you may recall sitting in a chemistry class and learning of the ideal gas law. This law states that pV = nRT. The pressure of the gas is given by p, volume by V, the amount of gas n, R is a constant and the temperature T.

Basically if you collapse out the amount and the constant you get an equation that says pV ∝ T. This means that if you increase temperature you also increase volume. If you seal the gas into a space to prevent the volume from expanding you get an increase in pressure instead. The same principle holds for liquids.

Why am I teaching a lesson on thermal expansion you may ask? Well, it is to explain the problem we’re having with our hydronic heating system.

A hydronic heating system has radiators, piping, a pump and a boiler. The boiler is like a BBQ that heats water that is circulated through the piping by the pump. The hot water flows through the radiators, and — through the holy triumvirate of conduction, radiation and convection — heats the house.

Water goes through the rads, and returns to the boiler to be reheated. When the thermostat stops calling for heat then the boiler no longer barbecues the pipes, and eventually the system water cools down and no more heat transfer takes place.

A key point about this system is that it is a closed one. Water stays in the pipes.

Now, we can tie together the first few paragraphs with the last few.

If you heat a liquid it expands. However, the hydronic heating system is full of water and the system is closed. This means that adding heat increases pressure since there is no room for expansion. Too much pressure is a bad thing. As any child who has overblown a balloon will attest to.

The system incorporates an overpressure valve that opens if the water pressure becomes too high. This valve has the positive effect of keeping the system pressure regulated and safe. However, it has the negative effect of making a mess in the basement.

In order to prevent the valve from having to open each time the boiler fires up (in order to relieve the pressure increase caused by thermal expansion) the system is equipped with an expansion tank.

Think of the expansion tank as the plumbing equivalent of a spring or a shock absorber. This tank is sealed, and essentially has a strong balloon inside it. When the water temperature (and hence pressure) increases it compresses the balloon, which then takes up less space and the tank accomodates the larger volume of water. When the water cools the balloon naturally increases in size due to the available space made by the decreasing liquid volume and the system is happy.

If, however, the balloon bursts inside the tank (or becomes deflated) then it no longer provides any cushion. In this circumstance, the tank fills with water and an increase in temperature has nothing to push against and nowhere to go. See above re: mess in basement.

This explains why I need to call a heating contractor tomorrow to blow up a balloon located in a sealed metal tank attached to my furnace.

    2006-10-30 21:40   ...0 comments

The lovely and talented eala_dubh (waves) posted a link to an OC Transpo driver’s comments on how to efficiently travel by bus in Ottawa.

    2006-10-30 14:47   ...0 comments

One of the things going on on the internet today is anonymous secrets.

It all started with PostSecret which involved people making artistic postcards and mailing them anonymously to a website.

Since then, all kinds of secret sites have popped up, like the ljsecret community on LiveJournal.

Amongst all of these is a site that lets you phone up, leave a message and the message is posted anonymously to the site as an audio file. This might be unremarkable except that the phone end of it is our PhoneValet application.

(The guy is providing an applescript to PhoneValet that is called after the message is left. The script post-processes the audio into MP3 format, then uploads it to his webserver.)

Cool to see a public use of PhoneValet, trivial though it may be.

ETA: The OneDaySecret site is here. Forgot that part. Click on the weird circle shapes to hear the secrets.

 › cool     2006-10-23 15:47   ...0 comments