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Wednesday, April 26, 2017

Evaporative vs Refrigeration cooling

Once upon a time I lived in the Mojave desert.  It's the most arid place that I've ever been to, although from what I've seen, it's nowhere near as dry as the Sahara desert.  In the summer months, the sun is intense and the heat is brutal.  Any time you live in a desert, it's a great idea to try to keep the inside of your house from getting as hot as the rest of the desert!

There are a few ways to cool a modern house.  One (very expensive) method involves burying a long loop of tubing several feet underground, where the soil is cool, and circulating water between the house and underground.  A heat pump uses this loop of water as the heat source or heat sink.  This works well because water has more mass than air, and it's quite efficient.  This method is called "geothermal heating/cooling".  The excavation for the underground pipe loop is hugely expensive, and therefore almost nobody installs it, so I'm not going to discuss it further.  The arrangement is below, in case you are interested in how it's done.

The second method for cooling a house in a crazy-hot environment is refrigeration - good old-fashioned air conditioning.  With this technique, we take advantage of a liquid that has a low boiling point to remove heat from inside the house and transfer it outside the house.   Below is a basic diagram of how an air conditioner works.
  1. A compressor pumps cool gaseous refrigerant up in pressure, which also causes it to heat up.
  2. A condenser receives the hot compressed refrigerant.  The refrigerant condenses into liquid, because a fan blows ambient air across the coils.  The condenser is located outside the house.
  3. The liquid refrigerant passes through an expansion valve (which is just a small orifice), and the pressure drops dramatically.  The sudden pressure drop causes the liquid to begin boiling at a very low temperature
  4. The boiling, low-pressure refrigerant enters the evaporator, where indoor ambient air is blown across the coils.  The heat added to the evaporator causes the refrigerant to boil off, while removing large amounts of heat from the air, as the refrigerant transitions into a vapor.  
Maybe you can spot the economic issue with using an air conditioner.  It needs three motors.  One motor runs a small fan that blows air across the cold coils inside your house.  Another motor blows air across the hot coil outside your house.  The compressor pumps the refrigerant gas up in pressure. This operation takes *a lot* of power, and the compressor uses the bulk of it.  Worse yet, the hotter the outside air temperature gets, the worse the condenser works.  Right when you need it the most, your air conditioner loses efficiency.

There's a better way to cool your house if you live in an arid or semi-arid place, and that's evaporative cooling.  In earlier days these were cheap window-mounted boxes called 'swamp coolers'.  Today they are built with improved materials and controls, are ducted into the home's central heating/cooling system.  They are now known as 'evaporative cooling systems'.

Let's check out the early technology first.  A swamp cooler is a way to cool your house in a less power-hungry way than air conditioning.  It uses only one fan, which requires only about one horsepower.  A small pump circulates water over a mesh media, and disposes of a small side stream of water down the drain, to avoid concentrating minerals as the water evaporates.

Below is a cutaway of a simple roof-mounted swamp cooler.  This one blows cool air downward:

So, the motor drives the fan, which pulls in a large volume of dry outside air and directs it downward into the house.  Before the air enters the fan, it enters the cooler through louvered panels and passes across wet mesh pads.  A portion of the water in the mesh pads evaporates, causing heat to be removed from the air.  The heat removal is due to the heat of vaporization of water, which is 970BTU/Lb.

The pump circulates water into V-shaped troughs that have tiny holes drilled in them to distribute water evenly across the top of each pad.  A float valve (similar to a toilet tank) keeps level in the pump basin constant. A bleed line continuously blows down a fraction of the water to prevent concentration of minerals that would otherwise cause scale to build up on the mesh pads.  Scale occurs anyway, but at a much slower rate than it would without the blow down.

The controls for one of the inexpensive units are "off" and "on".  If the unit is more expensive, controls may be "off", "low speed" and "high speed" for the fan.

These are simple machines.  They work great, aren't terribly expensive, and are cheap to operate.  Best of all, they are most efficient when the air is hot and dry :)  But they can also be improved on!

A more modern version of this device:

With this design, the big improvement is the wet media.  This is a thick section of cellulose, with much more surface area than a mesh fiber pad has.  The evaporative efficiency is much better, because air molecules are unlikely to pass through without coming into contact with water.   Below is a cutaway of the wet media:

Another huge improvement on a modern evaporative cooler is controls.  It can be operated using the central air system thermostat.  If the temperature exceeds the thermostat's setpoint, the evaporative cooler will turn on at low speed, which is the most economical.  If the temperature climbs too much above the setpoint, the evaporative cooler will kick into high speed to try to get your house cooled down again.  The unit will then either cycle between off/low or low/high depending on the cooling needs of the house.

Furthermore, the blowdown is handled by a timer, and the cooler will blow down intermittently, based on run time and fan speed.  This is done to avoid unnecessarily wasting water in a place that probably doesn't have much water to begin with.

Lastly a swamp cooler does something that an air conditioner will not:  It cools your attic down, thereby helping to reduce the amount of heat conducted indoors.  Attic temperatures can reach 140 degrees F, or 60 degrees C.  That's pretty hot, and it can heat up your ceiling and the inside of your house.

Because a swamp cooler draws in air from outside your house and forces it inside, you need to provide a path for air to leave your house.  The best place to release air is at the top of each room, where the hottest air rises to, yes?  You simply install an up-duct to release the warmest air from inside your house into the attic.  This improves home cooling efficiency by lowering the temperature in the attic while removing the hottest air from inside the house.  Clever as heck!

 Below, an up-duct. The damper opens when the evaporative cooler begins flowing air into the house, and closes back down when it shuts off.  There is a slide damper that will be inserted when cooling season ends, to prevent heat losses during heating season.  These also have a temperature-activated closure device in the event of fire.

A few words about swamp coolers and evaporative coolers.  They work wonderfully when the air is dry.  Once relative humidity climbs much above 25%, they stop evaporating water well enough to do a good job of cooling the outside air.  During the monsoon season, conditions become hot and humid, and an evaporative cooler doesn't do much cooling at all.  It's best to relax inside your RV and enjoy the refrigeration!

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