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Posted by daestrom on June 13, 2005, 8:25 am
>
> |> You are being silly. If you have a fixed capacity window unit you can
> |> size
> |> it to be "too big" all the time (and play games with fan speed to
> adjust
> |> capacity)
> |
> | No, as yo already pointed out, it is a fixed capacity, not variable.
> You
> | are deluding yourself to thing you can change the capacity. Sort of
> like
> | people that keep pushing up the thermostate to get the house heated
> faster
> | when the heat is already running. The copressor is either on, or off.
> | There is no varialble capacity.
>
> It is possible, though not always efficient, to vary the capacity of a
> heater. Two elements connected in parallel could be reconnected in
> series to have a lower temperature when the difference between desired
> and actual temperature of the air is small (1 degree).
There are also solid-state proportional heater controls, that use
phase-controlled thyristors to vary the heat output over a continuous range.
Usually not cost effective for simple space heating, but often used where
more precise temperature control is needed.
>
> Is there anything at all that can make the A/C coils be less cool than
> they would be at "full tilt"? One reason I want to know is that I see
> many cases of central A/C units freeze up; the coils are obviously very
> cold and apparently there is too much water and/or not enough air.
>
There is a lot I could go into in this area. The base temperature of the
evaporator is determined by the saturation pressure of the refrigerant
inside it. Advanced systems use a variety of suction pressure controls to
maintain this saturation condition just above 32F. In a high-end system,
suction throttle valves (a form of 'back-pressure' regulator) will maintain
the saturation temperature in the evaporator at between 35 and 45 F. This
is cold enough to condense a lot of the water vapor, but will still leave
some in the air. When air is at 100% RH at 40F, and then warmed back to 70
F, it will be about 50% RH.
Another pressure control method is various forms of 'staging' in the
compressor, or the use of variable speed compressors. By controlling the
amount of pumping the compressor does, the evaporator pressure is kept above
freezing. Interestingly, in modern auto A/C, there is a suction pressure
switch that cycles the compressor on/off by engaging/disengaging the clutch
used to drive it. Of course, cycling a motor driven compressor on/off is
not viable as it would have to do this several times a minute, and the
repeated high starting currents will burn out the motor.
Contrary to urban myth, the TXV (ThermoeXpansion Valve) is *NOT* used to
prevent freezeups. It works by sensing both the pressure and temperature at
the evaporator outlet, and is used to control the amount of superheat in the
refrigerent as it leaves the coil. A small amount of superheat is desired
to prevent the compressor from ingesting liquid (that would damage the
compressor). But amateurs will often 'fiddle' with the TXV when either the
unit freezes up or doesn't cool properly.
One cause of freeze-ups is too little charge in simple capillary units (no
TXV). The lower pressure in the system causes the evaporator to run at
lower temperatures.
And finally, despite what I wrote above, there *are* some cheap units that
are designed to have the evaporator running below 32F. These cheap units
rely on a high air flow to keep the evaporator warm enough to avoid a build
up of frost. They don't work well in humid climates. The best you can do
with these is to make sure the air flow is unobstruted by dirty filters.
daestrom
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