How Air Conditioners Work

AND

How to SAVE MONEY by making them more efficient!

(Portions reprinted from How Things Work.com)

When the temperature outside begins to climb, many people seek the cool comfort of indoor air conditioning. Air conditioners are one of those things that we see every day but seldom pay much attention to.

Wouldn't it be nice to know how these indispensable machines work their magic?

Air conditioners come in various sizes, cooling capacities and prices. One type that we see all the time is the window air conditioner, an easy and economical way to cool a small area:

People who live in suburban areas usually have a condenser unit in the backyard and

if you live in an apartment complex, you'll probably see multiple condensers for each dwelling:

Most businesses and office buildings have condensing units on their roofs, and as you fly into any airport you notice that warehouses and malls may have 10 or 20 condensing units hidden on their roofs:

Even though each of these machines has a pretty distinct look, they all work on the same principles. In this article, we'll examine air conditioners -- from small to huge -- so you know more about what you're seeing. We'll also look at some new, energy-efficient cooling methods.

Air-conditioning Basics

­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.

Diagram of a typical air conditioner, clockwise refrigerant flow.

This is how the evaporation cycle in an air conditioner works. The outside compressor compresses cool Freon gas returning from the house, which also generates heat as a by product, and high-pressure Freon gas (red in the diagram above).

This hot gas runs through a set of coils to cool it down and condense into a liquid.

The liquid Freon then runs through an expansion valve and in the process it evaporates to become cold, low-pressure Freon gas (light blue in the diagram above).

This cold gas runs through a set of coils (the Evaporator) that allow the gas to absorb heat from air passed through it by the circulation fan, and cool down the air inside the building.

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, like this:

The cold side, consisting of the expansion valve and the Evaporator 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, pointed up to suck air in through the sides of the coil and blow it out the top, 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. Other than the fact that the hot and cold sides are split apart and the cooling capacity is higher, 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. In larger buildings and particularly in multi-story buildings, the split-system approach begins to run into problems. Either, running the pipe between the condenser and the air handler exceeds distance limitations, or the amount of duct work and the length of ducts become unmanageable. At this point, it's time to think about a chilled-water system.

 Chilled-water and Cooling-tower AC Units

In a chilled-water system, the entire air conditioner lives on the roof or behind the building. It cools water to between 40 and 45 degrees. This chilled water is then piped throughout the building and connected to air handlers as needed. There's no practical limit to the length of a chilled-water pipe if it is well insulated.

You can see in this diagram that the air conditioner (on the left) is completely standard. The heat exchanger lets the cold Freon chill the water that runs throughout the building.

 In all of the systems described earlier, air is used to dissipate the heat from the outside coil. In large systems, the efficiency can be improved significantly by using a cooling tower. The cooling tower creates a stream of lower-temperature water. This water runs through a heat exchanger and cools the hot coils of the air conditioner unit. It costs more to buy the system initially, but the energy savings can be significant over time (especially in areas with low humidity), so the system pays for itself fairly quickly.

 Cooling towers come in all shapes and sizes. They all work on the same principle:

A cooling tower blows air through a stream of water so that some of the water evaporates.

Generally, the water trickles through a thick sheet of open plastic mesh. Air blows through the mesh at right angles to the water flow. The evaporation cools the stream of water.

Cooling Towers

The amount of cooling that you get from a cooling tower depends on the relative humidity of the air and the barometric pressure.

For example, assuming a 95-degree day, barometric pressure of 29.92 inches (sea-level normal pressure) and 80-percent humidity, the temperature of the water in the cooling tower will drop about 6 degrees to 89 degrees. If the humidity is 50 percent, then the water temperature will drop perhaps 15 degrees to 80 degrees. And, if the humidity is 20 percent, then the water temperature will drop about 28 degrees to 67 degrees. Even small temperature drops can have a significant effect on energy consumption.

Economical Smart Choices

(Copyright 2009, Keith A. Richards)

This leads one to wonder how the average homeowner can benefit RIGHT NOW from modern cooling technology without having to replace their existing equipment.  

If your home is already equipped with a heat pump style air conditioner, standard split system for heating and cooling, or only a few refrigerated window units, you can benefit in much the same way big business does by taking advantage of the same cooling principles they do in their huge systems.

If you think back on the introduction to cooling tower technology normally used in chilled water cooling systems you will remember that the really big systems circulate chilled water throughout the building to be used for cooling. Evaporation from the cooling tower helps the compressor chill the water too.

Remember this diagram of the chilled water system?

Homeowners can take advantage or this principle too by simple flipping the chilled water system around and using evaporation to help cool the outside condenser. The condenser benefits in the same way by requiring less energy to keep you house cool and extending its useful life by not having to work so hard to do it.

The Concept

The Quickest, easiest way to introduce evaporative cooling to your air conditioner is to simply point a garden hose at it when the outside compressor comes on, but remember, evaporative cooling is mostly effective when the humidity is below 50-60%.  It would also make little sense to waste water when the condenser doesn’t need any help, like when the outside air temperature is only 75 to 80 degrees. After all, we need to conserve water too don’t we.

Considering these things, there are only a few ideal circumstances that tell us when to apply the evaporative cooling principle. Outside temperature above 75 degrees, humidity below 50-60% (obviously not raining), outside compressor running, and we should be careful not to use any more water than necessary for conservation purposes.

Remember this diagram of the usual Split System?

This is typical, if your air handler is mounted in the basement of your house, but most of us down south have them mounted in a centrally located hall closet. Usually it includes a gas or electric heating element. This drawing more accurately represents a heat pump installation.

 All we need to do to implement evaporative cooling is to provide water to the outside unit with the appropriate controls to only use as much water as necessary under the appropriate conditions.  

The same outside condenser with evaporative cooling system.

The simple addition of a 3/8” water line from the closest water hose faucet, an electric shutoff valve, and a temperature and humidity sensor, will reduce your cooling cost and extend the life of your outside condenser by up to 30% in the hotter months of the year! Optional washable filter pads enhance your savings and give you a means to protect your outside condenser from clogging with debris such as grass clipping and dirt that get sucked in every time your condenser fan comes on.  

How can you tell its working?  

Virtually all conventional outside condensers have 2 power lines and 2 Freon lines running to them from the house. There are the heavy duty 220V power cable, a small control wire from the thermostat inside the house that tells the condenser when to come on and go off, and two copper Freon lines.

One of the Freon lines has an insulation sleeve on it. This is known as the Suction Line or Low Side. It carries the Freon gas from inside your house out to the compressor to be recompressed. The Low Side is cool to the touch as it returns to the compressor and is about the same temperature as the air inside your house. In fact in high humidity, it may even have frost on it where it is exposed to the air.

 The other Freon line has no insulation wrap on it. It is called the High Side because it is under high pressure after having been compressed and carries the Freon liquid back into your house for cooling.

 Sounds backward doesn’t it! Freon works by the expansion principle. The Freon is hot because it has been compressed and the condenser coils haven’t totally cooled it because they are not very efficient at temperatures over about 80-85 degrees. When it reaches the inside evaporator coil in your air conditioning closet it is allowed to expand through the Expansion Valve into the evaporator coil. This very expansion process transforms the Freon from a liquid to a cold gas that absorbs heat from the evaporator coil and carries it outside to be dealt with by the compressor and condenser coil so the process can start all over again. This High Side line is EXTREMELY HOT to the touch when the condenser is running and the outside temperature is above 80-85 degrees. Touch it like you would an electric fry pan or get burned!

 Of course the hotter the day the closer to full-time you compressor runs and the hotter everything gets outside trying to cool down the inside. At some point the compressor starts running 100% of the time with no rest period. This is called 100% Duty Cycle. This is when your electric meter goes nuts and the air conditioner loses some of its efficiency which makes it work even harder and use even more electricity. As the temperature of the condenser and compressor rises the Freon in the condenser can begin to boil and the volume of liquid Freon that is actually getting to the evaporator coil in your house is interspersed with Freon gas bubbles which are less efficient at cooling.

 By applying the evaporative cooling principle to the outside condenser we can help the compressor and condenser from getting so blistering hot. We eliminate the possibility of the Freon to boil, we keep the compressor from getting to 100% Duty Cycle, and we avoid astronomical electric bills.

 Compressors that work at a maximum of 75% Duty Cycle live longer than the ones that are subjected to 100% Duty Cycles. Even the inside and outside fan motors last longer with shorter Duty Cycles, and we accomplish this with only a small amount of water, and only when the conditions are right for assisting the system with the Evaporative Cooling Principle.

Try this experiment yourself.

 Turn on your air conditioner and let it run for about 20 minutes. Go outside and locate the un-insulated copper High Side tubing at your compressor. VERY CAREFULLY sense its temperature without burning your hand.

 Now, turn you water hose on very slightly, just a trickle, and by placing your thumb over the end of it make a fine spray and direct it toward the outside of your condenser. Only spray a fine mist and try to keep as much of the condenser misted as possible for a few minutes. Now notice how much cooler that High Side line is! Remarkable isn’t it? That’s because the condenser has been made more efficient and the compressor doesn’t have to work so hard to get the job done. You may have even noticed that the system cycled off while you were misting water on it. That means you helped it reached the set point of the inside thermostat more quickly and efficiently.

That’s what our concept does. It provides a cooling mist automatically that helps air conditioning be more efficient, saves electricity, saves wear and tear on the compressor, only operates when it can do the most good, and all for the price of a little bit of water. And, we all know, water is much less expensive than electricity.

Here's a List of what you'll need from Home Depot!

Connect parts in the order listed starting from outdoor hose bib.

Orbit SunMate Zinc Hose Y With Shut-off  (Optional to allow hook-up of garden hose too.)
Model 58084 
$4.97/EA Each, Or Brass hose Y which is better.

Female Garden hose to 3/4" PVC pipe thread adapter (Brass in picture, PVC available)
About $4.00?

3/4" PVC nipple ( Whatever convenient length, gray in picture)
About $.62