How your 3-ton A/C could only be operating at One Ton (Awe, you're kidding;
NO I'm NOT!)
Only two of many diverse factors that can cause very low Btu/hr evaporator
heat-load absorption and transfer scenarios.
Every refrigerant has a "vaporization" heat absorption factor per pound.
This heat-absorption effect is rated in Btu per pound of refrigerant.
(Btu/lb); if the total heat-load of the conditioned space is known (in
Btu/hr) we can find the total number of pounds of ("evaporated") (R)
refrigerant that must be circulated by the compressor and metered through
the evaporator coil.
The closer that Btu/hr match is, the more efficient the system "can be."
Many other factors determine overall efficient A/C operation.
It requires a specific level of heat to (boil) evaporate a particular
refrigerant. The amount of the heat-load on the DX-coil determines the
maximum amount of (R) refrigerant that a DX-coil can evaporate. A TXV
(R) metering devices will better adjust to the heat-load and help
protect the compressor from flood-back. Flow-rator metering devices will
flood the coil and usually destroy the compressor!
Many older gas furnaces had small blowers and motor HP and unloaded at
higher static pressures, along with under sized ductwork. Couple those
factors with dirty filters that begin to by-pass lint and you have blower
wheel blades and evaporator fins and coils insulated and even blocked. You
end up with a coil that is not evaporating but little liquid (R) to absorb
latent and sensible heat from the air.
You could have a 3-ton DX-coil (36,000-Btu/hr) only boiling enough (R) to
absorb one ton (12,000-Btu/hr of heat from the air.
If it only boils one-third the (R) per hour it will only absorb and transfer
to the condenser 1/3rd the Btu/hr of heat to the condenser.
Here is another scenario with the same result:
The beer-can-cold boy installs a unit on a home that has a very hot
interior. This time the blower is producing too much CFM per ton of cooling.
So he thinks he has to get the suction line beer-can-cold and begins to pour
I the (R). Later when the interior cools down and then return air filter
begins to load-up, the coil becomes flooded liquid (R) begins. Same
scenario, DX-coil, two-thirds flooded and only one-third of liquid (R)
vaporizing to absorb heat. (A rated three tons, becomes a one ton operating
When the relative humidity is high, the ability of the evaporator to absorb
latent heat increases. Later, I will graph the increased ratio as humidity
goes up. The latent capacity of an air-conditioner depends on the specific
engineering of that specific evaporator and condenser match. In a 75-F
conditioned space you need 55-F discharge supply air over a period of long
run-time, mixing with the 75-F air to reach the 50% RH point.
- udarrell - Darrell
Air Conditioning System - Excessive Airflow - Excessive Charge