Why do portable ACs have filters for the evaporator but not the condenser? 3

[LMF5000]

I just bought my first portable air conditioner - a new Trotec PAC 2010SH. A curious design feature is that while the evaporator coils on the top have a conventional plastic air filter to filter air going through them, the condenser coils at the bottom don't have a filter - they seem to suck in unfiltered indoor air.

Why is this? Why not have filters for both? What makes the condenser able to function without filtering but not the evaporator?

This model has a heat pump so the roles are reversed in heating mode - the hot coil is filtered and the cold coil is not.

 

[Compositepro]

Good question. My guess is the the evaporator coil gets wet due to condensation, so it would tend to plug due to dirt sticking to the wet fins. The evaporator coil usually also has more closely spaced fins to promote dehumidification, which is not a factor with the condenser. The main failure mode of these types of air conditioners (including window units) is plugging of the evaporator with dust and algae.

 

[LMF5000]

Thanks, I figured as much - but what about when it is working in heating mode? Then the cold coils will be the unfiltered ones. Or perhaps it doesn't get so wet due to the greater fin spacing?

Why does closer fin spacing promote more dehumidification?

 

[3DDave]

To dehumidify the water molecules need to be in contact with the metal or already condensed water. Otherwise they condense on airborne particles and remain in suspension and produce fog which will soon evaporate and return to being humidity again. Smaller spacing = more fins = more area for the cooled water molecules to collide with the fins or previously condensed water.

 

[lilliput1]

The filter is for the air supply going into the room. The condenser coil (or cooling coil in reversed heat pump operation) discharges air to the outdoors so it is not filtered.

 

[Compositepro]

Seems plausible, at first, liliput1. But without the filter, the wet evaporator would still catch most of the dirt. So the filter is to protect the evaporator, and not to filter the air for the occupants of the room. Room fans do not have filters because they do not need to be protected, just as the dry condenser does not need protection.

 

[LMF5000]

Which takes us full circle to my second question - in "heat" mode everything is reversed - the cold coils are the unfiltered ones. Why was it designed like that?

Is it that the design of the unfiltered coils (larger area and more spacing) makes them not cool too much in heat mode so there's no condensation on them? Or does a portable AC actually destroy itself with wet dust in heat mode, but it was considered an appropriate design decision to save the $10 filter?

 

[Compositepro]

I can only speculate. Filters are designed to get plugged and require regular maintenance. They are to be avoided if possible. The "outside" coil has a larger fin spacing because it is not intended for dehumidification, so dirt can pass through. When in heat mode, the "outside" coil temperature will likely be below freezing, causing frost that can be blown-off, rather than condensation that causes dirt to stick. Absolute humidity outside on cold days will be low.

 

[EnergyProfessional]

Regular split systems (condenser outside, separated from the indoor evaporator) also don't have filters on the condenser and need somewhat regular cleaning. This depends on what dirt and plant items are flying around. But it is common to at least annually spray them out with the water hose. that still is less maintenance than filters would need. and an outdoor unit also gets wet. So adding a cloth-or paper filter would require quite some large construction to actually work in all weather conditions. You also add fan energy required when you filter. So you only filter when the benefits outweigh he cost.

 

[IRstuff]

Can't imagine any universe where AC condenser would have a filter; you want BOATLOADS of air on the condenser to get the cooling maximum efficiency. Sticking a filter in way is inefficient and gosh-darned expensive if you want comparable air flows.

TTFN (ta ta for now)
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[SAK9]

Condenser and evap cooling have different fin spacings condenser having greater fin spacing.Therefore condenser coils are more tolerant of dust.It is also impractical to have filters on condensers as they will choke up in no time leading to their degraded performance.