Optimizing Swamp Cooler Performance

Swamp Cooler Physics

How to Optimize Swamp Cooler Performance

When the outside air was 102°F, my swamp cooler was making 69°F air: 33°F cooler. If your evaporative cooler doesn’t perform like this, consider the following approach.

Swamp Cooler Basics: Theory, Structure, and Why They Don’t Always Work

  1. Cooler = the swamp cooler, House = the space to be cooled, “” means “implies”.
  2. Swamp cooler structure:
    1. A swamp cooler is a 3′-4′ hollow cube of sheet steel, connected to the house by a duct. The cube holds a big centrifugal air blower, a 1-hp electric motor to run it, and a centrifugal water pump.The box sides are removable metal frames, each filled by a 2″ thick pad of long, thin aspen shavings. A removable wire bail holds each pad in its frame. The outer face of each frame is louvered so air can pass through the pads. A horizontal channel, V-shaped in cross section, lies along the top of each frame. The side of the V-channel facing into the box has slots every 2″.
    1. The box bottom is the swamp: a ~3″ deep reservoir of water, in which sits the water pump.
    2. The fundamental principle of swamp cooling:
      1. Swamp coolers are evaporative coolers: they cool by changing water liquid into water vapor (i.e., evaporation). The design intent of a swamp cooler is to evaporate as much water as possible.As water evaporates, it removes significant heat from the environment. Simply cooling 1 cc of water by 1°C (e.g., 50°  49°) removes 1 calorie of heat. But evaporating 1 cc of water removes 540 calories. Thus evaporating a cc of water removes 540 times as much heat as cooling that cc of water by 1°C.A swamp cooler can cool air by as much as 35°F. More water evaporated = more heat removed.
      Evaporativecooling is effective only when the air is very dry; say, when its relative humidity is below 10%, although some cooling occurs up to about 25%. Humidity > 25%  Poor evaporation  Swamp coolers don’t cool well. Sad to say, the only way to reduce the outdoor humidity is to change the weather.
  3. Ideal swamp cooler operation: a., b., c. Anything that keeps a., b., c. from happening is a problem.
    1. The swamp is full of water. A float valve connected to the water supply meters water into the swamp.
    1. The aspen pads are soaking wet. The water pump pumps water from the swamp through plastic tubes up to the channels atop the frames. The water trickles through the channel slots onto the pads below.
    1. The blower pulls hot, dry, outside air through the wet pads. This evaporates large amounts of water, humidifying the hot dry air and cooling it by as much as 35°F.
    1. The blower forces the cool wet air into the house. The incoming cool, wet air pushes the hot, dry air in the house out through open windows and vents. constantly replacing the hot air with wet air. The blower should move enough air—at least 3000 cfm—to replace all the air in the house 20 times an hour.

General Swamp Cooler Monitoring and Maintenance

  1. Once a year in mid-May, start your cooler and re-acquaint yourself with it. Take each pad frame off, then watch and listen. How does it sound? What’s the belt doing? Where’s the water going? Are all the pads wet?
  2. Find a place where you can hear the cooler running. Once a week, go listen to it run for a few minutes. If it starts to sound odd, inspect it. Odd noises imply unusual events, and Murphy’s law suggests that unusual events are problems. Here’s a short list of possible problems that you will almost certainly be able to hear:
    1. The pump is not getting enough water. Why? Is the cooler leaking? Is the supply line clogged or leaking? Is the water supply scanty? And does the pump work when it is getting enough water?
    1. The pump bearings are failing. When they get worn, the pump will start to squeal and make grinding or rubbing sounds. A/C water pumps use cheap, sealed sleeve bearings that cannot be replaced, serviced, or lubed. To make up for this, a new centrifugal water pump is only $15-$25. Or the pump may just stop.
    1. The belt driving the blower is worn or broken. Take it to an auto parts store (trust me) and replace it.
    1. The blower bearings need lubrication (they can be lubed) and/or are worn and/or are failing.
    1. The most awful noise is no noise: The motor’s not running. Either it’s shot or the electricity has failed. If you can hear neither pump nor motor, check the circuit breaker (the pump might be shot, but simultaneous motor failure is unlikely). If you can hear the pump, prepare to spend $100-150 for a new electric motor.
  3. Visit your cooler once a month during cooling season. Most are on the roof so gravity can help the heavy cool air made by the cooler fall into the house. Strive for one trip; you don’t want 5 or 6 trips up and down the ladder. Get a canvas bag and put in pliers, a “- ” open-end wrench, a jackknife (Swiss army knives are useful), Philips-head and regular screwdrivers, a tubing snake, a small bottle of Clorox, and blower bearing lube. Start the cooler, get the canvas bag, and go up on the roof. Lube the blower bearings. Put a few tbsp of Clorox in the blower pan. Find and correct leaks, dry pads, meager and misdirected water flow, water system clogs, a loose blower-drive belt, a float valve that doesn’t shut off or open up, algae- and caliche-clogged pads (replace them), and pad debris in the swamp (if you find any, get it out and leave it on the roof for the wind to blow away).

Maximizing Swamp Cooler Performance

  1. Problem #1: Air coming out of the cooler is not cool. Possible issues:
    1. The outside air is too humid. Cooling ability drops as humidity rises. Not much can be done.Insufficient water (pads are not wet enough: they must be soaking wet and dripping)Insufficient air movement (blower isn’t running; air flow is blocked or diverted; belt is loose)
    1. The air isn’t passing through wet pads. The pads are wet and the air is moving, but it’s not cooling.
  2. Problem #2: The cooler is making cool air, but the house doesn’t get cool. Possible issues:
    1. Hot air in the house must be replaced by cool air. Primarily, the air in the house needs to be let out.
    1. When it’s hotter outside than inside, the air in the house heats up. A swamp cooler pushes cool air into the house, displacing the warm air and pushing it outside. When air flow is managed well, hot air in the house is continuously replaced by cool air; when managed poorly (or if it’s humid) this doesn’t happen.
    1. Insufficient air movement (Air flow is blocked, diverted, or poorly managed inside the house).
    1. An attic full of hot air will heat the whole house. Vent the attic. If you don’t have an attic, rest easy.
  3. Use aspen pads. Don’t bother with cardboard, paper, or plastic pads. I have tried them.
  4. Aspen pads absorb water. They must be soaking wet: lots of water must evaporate. Each holds a quart+ and weighs 2-3 lbs. more wet. If the pads grow algae, your house will smell like a lake. Put two tbsp unscented chlorine bleach in the swamp. Twice a year (May and July, say), replace the pads. Each is $5-6.
  5. Cardboard and paper pads quickly collapse into mush that can’t be redistributed. They clog, collapse, leave gaps, and need replacement—a very messy job—about once a month. Each is $7-8.
  6. Plastic pads don’t cool well because they don’t absorb water. A plastic pad in use for a week will weigh about the same as a brand-new one. Each is $10+.
  7. Open the windows: The cool air being pushed into your house must have somewhere to go.
    1. Refrigerated cooling: Windows must be closed. Air is not exchanged between outside and inside: hot air stays out, cool air stays in. A refrigeration unit cools warm air taken from the house and returns it to the house. The unit sits outside the house and vents waste heat.
    1. Evaporative cooling: Windows must be open. Air is exchanged between outside and inside: hot air is drawn from outside, cooled, and blown into the house. Air must exit the house to make room for cool air.
  8. Measure hot and cold. This is crucial. Buy 1 good thermometer for calibration and 4 or 5 cheap ones. Get some you can relocate and read from a distance.
  9. Incoming cool air must be managed. Easier when the house has built-in ducts; harder when all the cooled air enters through a big hole in the ceiling. Warm areas heat up the cool areas, so include them in the overall strategy or close them off.
  10. With evaporative cooling, the only way to cool an area is to move cool air through it. Controlling air flow is essential; windows and fans are the control devices. Bear in mind you’re cooling the entire house: You won’t instantly feel cooler when you change the air flow; a few degrees is a big improvement. Keep an eye on the thermometers and be patient.
    1. Fans: Use fans to blow cool air towards warm spots. For windowless rooms, use fans to push cool air in; warm air will exit. Cool, dense air will stay low; use fans on the floor to blow cool air up and out. If cooled air enters your house via one large opening, put fans near that. Close the doors of rooms not in use. If you have a duct opening between furniture and a wall, put a fan back there to move air into the room.
    1. Windows: Open every window an inch. Incoming air will push the warm air out through the openings.
      1. If a room gets hot, e.g., when the oven is on, increase airflow through that room by opening its windows an extra inch. Once the room cools off, put them back the way they were.
      1. Total open area for all windows should be ~1 yd2 per 2000 ft2 of floor. If you exceed this, the whole house will get warm: overpressure can’t be maintained and hot outside air will come in the windows. If this happens, close all the windows by ¼” (e.g., 1″ opening  ¾”) and watch the thermometers.
      1. Security: Don’t let the windows open more than a few inches. Make window stops from ½”x1″ lath or ¾” dowels cut to length. For casement windows, take the handles off so they can’t be opened or closed.
    1. General: Relocate stuff that’s in the air flow: stuff hanging in doorways, over vents, and in front of windows; clothes, towels, drapes, open doors blocking floor vents, plants, piles of books, etc. Clear all duct openings, window openings, and spaces above/below/in front of these openings. Why? Air flow at the openings is roughly laminar; stuff turbulates it. Turbulence  friction  heat  moving the air takes more energy  the air slows down and heats up.
  11. Work on one room at a time, outward from the center. Start with the main living space; when it’s cool, go for the kitchen. Keep the windows, doors, and ducts closed in rooms you haven’t reached yet. You should be able to get the center cool quickly, then work outward from the most-used spaces to the least used. Be more patient as you move outward: You’re using the same amount of incoming air to cool an ever-greater volume.
  12. Evaporation—and thus cooling­—is maxed when all inbound air goes through wet pads.
    1. Ensure all louvers are backed 100% by full-depth pads. Again, all inbound air must go through wet pads.
    1. Oversize pads are ideal. Go for 1-2″ extra on all frame edges and cram them in with a putty knife. Don’t overdo it; they get wet and swell up. But don’t be neat: more aspen = more water = better cooling.
    1. Don’t, however, double the pads. 2 sets will impede airflow once they get soaked and swell up.
    1. Air going through wet pads is cooled; air going through gaps in the pads is not. And inbound air prefers gaps because they don’t restrict air flow like wet pads do. Both effects reduce the percentage of air that gets cooled. Finally, hot air coming in through the gaps heats the cool air before it even enters the house.
    1. Close gaps by re-arranging the aspen under the wire bails. If gaps remain, get new pads.
    1. Old, sodden pads that have been in use too long will disintegrate and collapse inside the vertical frames. A frame can be 1/3 empty because of this, and a collapsed pad blocks almost 100% of the air flow. In this situation, most of the incoming air never contacts a wet pad and is not cooled at all.
  13. Ensure that all aspen pads are soaking wet when the cooler is running. The cooler cannot cool when the pads are dry. Strive to comply with the following, ensuring water flow as you go:
  14. Dry pads reveal a water supply problem. Small dry areas are OK; a wet pad can look dry where the surface water has evaporated. But a bone-dry pad is a problem.
  15. A working pump moves a lot of water. Watch it run. Ensure copious water flow through all water lines. Ensure that the pump sits in a basket or screen that’s free of debris and dirt. A 2-year life for a cooler pump is not bad. When replacing a pump, buy two; be ready for the next pump failure. They cost $15-25.
  16. Ensure that all water lines are open and not clogged with algae, pad debris, insects, etc. Buy a spider snake ($5-7): a thin, two-foot long spring on a handle. Use it as you would a pipe cleaner to probe and clean the water lines.
  17. Ensure there’s plenty of water in the channel along the top of each pad frame. Clean all the slots in each channel with a knife so water runs through them freely.

Turbulate is a verb meaning “to cause a flow to transition from laminar to turbulent.”