Hot And Busted

No, it's not the name of a new supermodel reality show-- it's the air conditioning: hot and busted.

Of course these things only break when you use them - which, of course, is precisely when you need them. It's been hot, humid, and generally miserable all week. It is 90-100F during the day and it "cools off" to 80 at night.

To a mechanical ignoramus like me, I know one thing: it's busted. Other more enlightened souls may care about how this magic box called the Air Conditioner (AC) works.

From How Stuff Works:

Air-conditioning Basics

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Most people think that air conditioners lower the temperature in their homes simply by pumping cool air in. What's really happening is the warm air from your house is being removed and cycled back in as cooler air. This cycle continues until your thermostat reaches the desired temperature.

An air conditioner is basically a refrigerator without the insulated box. It uses the evaporation of a refrigerant, like Freon, to provide cooling. The mechanics of the Freon evaporation cycle are the same in a refrigerator as in an air conditioner. According to the Merriam-Webster Dictionary Online, the term Freon is generically "used for any of various nonflammable fluorocarbons used as refrigerants and as propellants for aerosols."

Diagram of a typical air conditioner.

This is how the evaporation cycle in an air conditioner works (See How Refrigerators Work for complete details on this cycle):

1. The compressor compresses cool Freon gas, causing it to become hot, high-pressure Freon gas (red in the diagram above).
2. This hot gas runs through a set of coils so it can dissipate its heat, and it condenses into a liquid.
3. The Freon liquid runs through an expansion valve, and in the process it evaporates to become cold, low-pressure Freon gas (light blue in the diagram above).
4. This cold gas runs through a set of coils that allow the gas to absorb heat and cool down the air inside the building.

Mixed in with the Freon is a small amount of lightweight oil. This oil lubricates the compressor.

Air conditioners help clean your home's air as well. Most indoor units have filters that catch dust, pollen, mold spores and other allergens as well as smoke and everyday dirt found in the air. Most air conditioners also function as dehumidifiers. They take excess water from the air and use it to help cool the unit before getting rid of the water through a hose to the outside. Other units use the condensed moisture to improve efficiency by routing the cooled water back into the system to be reused.

So this is the general concept involved in air conditioning. In the next section, we'll take a look at window and split-system units.

Window and Split-system AC Units

A window air conditioner unit implements a complete air conditioner in a small space. The units are made small enough to fit into a standard window frame. You close the window down on the unit, plug it in and turn it on to get cool air. If you take the cover off of an unplugged window unit, you'll find that it contains:

• A compressor
• An expansion valve
• A hot coil (on the outside)
• A chilled coil (on the inside)
• Two fans
• A control unit

The fans blow air over the coils to improve their ability to dissipate heat (to the outside air) and cold (to the room being cooled).

When you get into larger air-conditioning applications, its time to start looking at split-system units. A split-system air conditioner splits the hot side from the cold side of the system, as in the diagram below.

The cold side, consisting of the expansion valve and the cold coil, is generally placed into a furnace or some other air handler. The air handler blows air through the coil and routes the air throughout the building using a series of ducts. The hot side, known as the condensing unit, lives outside the building.

The unit consists of a long, spiral coil shaped like a cylinder. Inside the coil is a fan, to blow air through the coil, along with a weather-resistant compressor and some control logic. This approach has evolved over the years because it's low-cost, and also because it normally results in reduced noise inside the house (at the expense of increased noise outside the house). Other than the fact that the hot and cold sides are split apart and the capacity is higher (making the coils and compressor larger), there's no difference between a split-system and a window air conditioner.

In warehouses, large business offices, malls, big department stores and other sizeable buildings, the condensing unit normally lives on the roof and can be quite massive. Alternatively, there may be many smaller units on the roof, each attached inside to a small air handler that cools a specific zone in the building.

Give Me A Lift

A couple of weeks ago, I had a mishap with the lawn tractor which rendered it unserviceable.

I still don't know exactly what's wrong with it, but I do know the mowing deck lift mechanism is damaged. It now has two positions: all the way up, or all the way down. Such is the way when you hit a tree root.

In a let's-pass-the-bill-to-find-out-what's-in-it fashion, I bought a tractor lift so I can find out just what is ailing my mower. It's just simple physics, really - a giant lever to lift the front end up so you can look underneath. It just helps that this lever is a tempered steel construction.

[WP] The magnitude of torque depends on three quantities: First, the force applied; second, the length of the lever arm^[4] connecting the axis to the point of force application; and third, the angle between the two. In symbols:

where

τ is the torque vector and τ is the magnitude of the torque,

r is the displacement vector (a vector from the point from which torque is measured to the point where force is applied), and r is the length (or magnitude) of the lever arm vector,

F is the force vector, and F is the magnitude of the force,

× denotes the cross product,

θ is the angle between the force vector and the lever arm vector.

The length of the lever arm is particularly important; choosing this length appropriately lies behind the operation of levers, pulleys, gears, and most other simple machines involving a mechanical advantage.

I put it together tonight and can't wait to use it. It is a very manly activity (grunt, grunt). Everybody should have a tractor and a lift to go with it.

Not to be outdone, however, Blynken came out from her bed tonight and wanted to know what "that smell" was.

"It smells like the lawn mower."

"It is the lawn mower - or at least a lawn tractor lift.", I said. "It smells like industrial grade metal, plastic, and rubber."

"I love that smell, Daddy."

Whatever.

Hacking The Pump?

Ever wonder how a gas pump works? They are fascinating pieces of technology that we all but take for granted. They mechanics are cool, but ultimately not that complicated. These days the "important" parts are done by computer. (The important part being paying for it.)

[HSW] As the gasoline travels upward into the dispenser, it passes through a flow control valve that regulates the gasoline's flow speed. It does this via a plastic diaphragm that gets squeezed more and more tightly into the pipe as the flow of gas increases, always leaving just enough room for the proper amount of gasoline to get through.

This pipe also contains the flow meter, which is a cast iron or aluminum chamber containing a series of gears or a simple rotor that ticks off units of gas as they pass through. Information about the gas flow is passed on to a computer located in the dispenser, which displays the metered amount of gas in tenths of a gallon.

In the 1970s, glowing LCDs in the form of seven-segment displays began to appear. (The segments in the display could be illuminated by computer to form various numerals and occasionally letters of the alphabet.)

These relatively simple user interfaces are gradually being replaced by full-fledged computer video displays, many running variations on operating systems like Microsoft Windows. These displays can offer information, display the amount of gas being sold and even run advertisements and carry on simple conversations with amused customers.

I actually find these computer / video displays really annoying. I just want to pump gas and leave. I don't want the news, the weather, or a car wash. Since I know a thing or two about computer [in]security, I'm always a bit skeptical about computerizing everything.

So imagine my somewhat amused surprise when I pulled in to fill up at the local gas shack and I see this:
If you look closely you can see this is a Windows computer. The task bar is the lighter bar at the bottom of the screen, the system tray is in the right hand corner, and there are 3 applications/windows open. There is a popup window in the middle of the screen with the classic "Do this / Cancel" combination. I wonder what it says?

If you squint hard (or click for larger image) at my still life with cell phone picture, you can make out the message "An updated version of LogMeIn has been downloaded and is ready for install. Click Update to install the new version now."

Remote access and desktop control to the gas pump? Interesting. Foolish, but interesting. Have these people never heard of EvilGrade, "a toolkit for exploiting products which perform online updates in an insecure fashion"?

Fake updates, plus remote desktop access to the gas pump. Wow. I'll bet nobody ever thought of hacking the pump before now.

It's not that far-fetched. Now I don't know if this computer is controlling the pump, or just the annoy-ware video, but it doesn't matter. I know you're sleeping better at night knowing that Micro$oft products may be controlling the gas pumps and your credit card number. I mean, what could go wrong?