Blow or Suck - refrigeration fanpenses of Cruising and Living on the Boat

[wingssail]

Originally Posted by Brewgyver

Turbulence = inefficiency.

Condenser fans always pull air through the coil. Is it possible that a unit exists that works the other way? Sure, but I've never seen one, and I've dealt with all kinds of air cooled units up to 100+ tons.

Virtually all window unit air conditioners blow air through the condenser and suck air through the evaporator.

[SV Bacchus]

Not to muddy the waters but.. When I had my fridge/freezer converted from cold plate to evaporator style everything was replaced except the BD50, which the installer/business owner said was working perfectly.

However, the gentleman said the fan was set up incorrectly. He said in colder climates people actually switch them seasonally. I don't remember the directions as it wasn't important to me being a Florida boat hanging out in the Eastern Caribbean. It blows, or sucks, in Summer and the opposite in Winter.

The downside to my setup is the unit gets air from the lazarette in the port sugar scoop. That is the same place my genset is located. The gentleman suggested I plumb cooler air via a tube to the BD50. The unit works extremely well without it and I wasn't keen to make a new hole anywhere, even if it was in a inside bulkhead.

He did replace my fan with a higher flow rate unit and switched the direction to what was recommended "for hot climates".

[Brewgyver]

Originally Posted by wingssail

Virtually all window unit air conditioners blow air through the condenser and suck air through the evaporator.

True, but in that case it's an expedient. They use one double-shaft fan motor and have to achieve flow over both coils with one motor and in as small a package as practical. The evap and condenser coils are both vertical, and at most only about 8-10 inches from each other. The evap fan has a molded styrofoam shroud. If the condenser side pulled air through it would be directing the heat in toward that shroud, adding heat back into the cooled air stream.

[sailorboy1]

Some of you should consider publishing papers to counter the other PhD papers out there. You know one of the ways you generally can tell if someone is wrong? They start using absolute words like "always" etc.

It was mostly a curiosity question and in the end I am pretty sure of the answer being that it is better to draw from which ever side of the condenser has the cooler air.

[Madehn]

how do cars with electric fans do it on their radiators-- if you have a transvers engine then you probably have electric fans moving the air- since they could mount it either in front of or behind the radiator, i am thinking they ( detroit engineers with huge budgets )would have used the most efficient way to get rid of the heat?

[S/V Adeline]

Originally Posted by Madehn

how do cars with electric fans do it on their radiators-- if you have a transvers engine then you probably have electric fans moving the air- since they could mount it either in front of or behind the radiator, i am thinking they ( detroit engineers with huge budgets )would have used the most efficient way to get rid of the heat?

That is actually the train of thought that caused me to reverse condenser and evaporator. On a traditional inline engine, the fan sucks air through the radiator and blows same air over the engine. With most early electric fans (mounted behind the radiator) they are doing the same. Later came electric mounted in front, blowing through. When the vehicle travels down the road, ram air is forced through the radiator and to operate the fan against that would defeat the purpose and quite quickly burn up the fan.

[Vinnie1966]

Hello everybody,

if the blower blows, it blows a bit of the heat generated by the fan motor. If it sucks, then this does not happen. Right?

cheers
vin

[Brewgyver]

Originally Posted by Vinnie1966

Hello everybody,

if the blower blows, it blows a bit of the heat generated by the fan motor. If it sucks, then this does not happen. Right?

cheers
vin

No, but also the answer isn't that simple. With a typical simple propeller fan, the fan blade attached to the motor shaft, the motor is effectively cooled by the air moved by the fan, no matter which way it blows. If you look at the specs of such fan motors you'll find "Air over" meaning they need to be in the airstream to prevent overheating. There are also propeller fans that are belt driven, with the motor typically out of the path of flow, as in a Whole House Fan:

They typically have motors with internal fans, as do motors that are generally used for other purposes, such as pumps.

[thinwater]

Originally Posted by guyrj33

i've been told it's more efficient to pull air than it is to push air.

This is true of pumps, which can cavitation. But at the pressure differentials were are talking about here, it makes no difference. Also, as others have pointed out, fan motor cooling and exposure to dust and condensate must be considered.


You need to consider the whole unit design. No one answer.

[David B]

Originally Posted by sailorboy1

It was mostly a curiosity question and in the end I am pretty sure of the answer being that it is better to draw from which ever side of the condenser has the cooler air.

Correct in my book. With a typical remote compressor/condenser unit, if you have the fan sucking air through the condenser, it will be blowing that hot air over the compressor (which without fins, has very little surface area to dissipate heat anyway).

If you have the fan blowing air through the condenser, it will be taking that air across the hot compressor. Good for the compressor, not as good for the condenser, but unless the ambient is really high (and the condenser needs all the cooling it can get), then that seems the best compromise.

Better - significantly more airflow, and duct the hot air overboard (or keel cooling).

In our case, the freezer unit (Vitrifrigo) blows through the condenser, and the hot air exhausts straight into the galley under the oven onto my First Mates ankles. Nice in winter, but we sail in summer - in the Med.
Ask me why I am considering changing this ........

[GordMay]

Standing alone (there’s more to this, but), the blow side of a fan produces a more concentrated, faster moving, and more turbulent airstream, compared to the intake side, where air is drawn almost equally from all directions. The velocity of the air downstream of the fan is roughly twice that of the air passing through the fan.*
The turbulence also greatly improves the efficiency of the heat transfer. Turbulence is in fact your friend.
So from those points of view alone, the blow side does appear the better cooling side.
If you're trying to ventilate (cool) an entire room/compartment), exhausting (sucking out) air is more efficient.

* Have you ever tried to suck out a candle?

[SV Bacchus]

Originally Posted by sailorboy1

It was mostly a curiosity question and in the end I am pretty sure of the answer being that it is better to draw from which ever side of the condenser has the cooler air.

Which, I believe, may be the reason the tech in Grenada said in some climates it is best to switch the direction of airflow twice a year as noted in my previous post.