For equivalent performance, it seems we also have to add a motorized
bypass damper to the swamp cooler to allow indoor air recirculation.

Maybe not, for an indoor swamp cooler, eg the WisperCool P300 ($154 at
Wal-Mart, but no longer being made) or the $298 Mastercool Mobile MMB10
(Grainger 5MU36), which has a garden hose connection and draws 3.5 amps
at 120 V. Adobe Air says it can cool 2000 cfm of 110 F 10% RH air 32 F,
about 69K Btu/h (6 tons), like 14 5K Btu/h window air conditioners :-)

For more cooling capacity with dry outdoor air, we might put one near
a window inside a house with a $55 Lasko 2155A 16" 90 W 2470 cfm intake
fan in the window and use the fan thermostat to turn on the cooler when
the room temp rises to 80 F and a humidistat to turn on the fan when
the indoor RH rises to 56%, with 1-way plastic film dampers in a box
between the cooler and the window to force outdoor fan air to flow
through the cooler pad when the window fan is running and make indoor
air flow through the cooler when the window fan is not running, like
this, viewed in a fixed font like Courier:

| |
| |
---------
| |llld| |
|c| d| | outdoors
|o| d|f|
==|o| d|a| == With the window fan off, indoor air
|l| d|n| would flow in through left and right
|e| d| | dampers lll and rrr. With the fan on,
|r| d| | ddd would open and the fan air would
| |rrrd| | force lll and rrr closed.
---------
| |

We might have 4 modes:

80 F 56%| window fan cooler fan cooler water
--------------|-----------------------------------------------
1. no no | off off off
2. no yes | on off off
3. yes no | off on on
4. yes yes | on off on

Case 3 would maintain indoor comfort with less water than an external
swamp cooler, for a house with significant natural air leakage, ie
for almost all houses.

Nick