depth of a hot air box

[69Cyclone]

Hi
Newbie here

Is there any advantage to a deeper box to make up for lack of over all glass? I have seen then the depth of the can mostly.
Does more volume inside help?

thanks
Joe

[Wy_White_Wolf]

IMHO

Increased depth only helps up to the point that the depth no longer hinders thermal airflow of the box. After that point increased depth can hurt heat gain by allowing for a thermal cycle occurring inside the box (hot air in the back of the box rising while cold air next to the glazing falls).

WWW

[69Cyclone]

IMHO

Increased depth only helps up to the point that the depth no longer hinders thermal airflow of the box. After that point increased depth can hurt heat gain by allowing for a thermal cycle occurring inside the box (hot air in the back of the box rising while cold air next to the glazing falls).

WWW

IMG_3577.JPG


This is my 1st attempt.

thanks

[J.P.M.]

If I understand what you're writing and attempting correctly, keep the glazing-to-back of the box dimension as small as possible/practical. Doing so will increase the velocity of the air, and the heat transfer coefficient between the inside surface of the box and the air will improve. While you're at it, put a diffuse in the inlet if you don't have one there now. Also, some dark (black) aluminum mesh, or some material that can take elevated temps. and stay dimensionally stable will improve heat transfer. Just don't pack it so tight that air flow is impeded by increased pressure drop and thus the increased temp. rise decreases thermal efficiency.

[69Cyclone]

If I understand what you're writing and attempting correctly, keep the glazing-to-back of the box dimension as small as possible/practical. Doing so will increase the velocity of the air, and the heat transfer coefficient between the inside surface of the box and the air will improve. While you're at it, put a diffuse in the inlet if you don't have one there now. Also, some dark (black) aluminum mesh, or some material that can take elevated temps. and stay dimensionally stable will improve heat transfer. Just don't pack it so tight that air flow is impeded by increased pressure drop and thus the increased temp. rise decreases thermal efficiency.

On mine the depth is 12". I did not intend to have it like that as I changed designs midstream. I used the lath / plaster screening and I added 4 large ceramic tile as an add on to the collector. My box is very free flowing. I have seen temp of 170 thus far. I need to square away the fan and snap switch and plumbing inside my garage to see how effective it is.

solar 2.jpg

[J.P.M.]

On mine the depth is 12". I did not intend to have it like that as I changed designs midstream. I used the lath / plaster screening and I added 4 large ceramic tile as an add on to the collector. My box is very free flowing. I have seen temp of 170 thus far. I need to square away the fan and snap switch and plumbing inside my garage to see how effective it is.

[ATTACH]7920[/ATTACH]

The goal is usually quantity of heat, not quality (high temp.) which, beyond what's required, actually reduces efficiency. Get more air through the box - probably a bigger fan.

[69Cyclone]

The goal is usually quantity of heat, not quality (high temp.) which, beyond what's required, actually reduces efficiency. Get more air through the box - probably a bigger fan.

Quantity meaning flow and temp? Without a flow meter is there any other method to record this?

[J.P.M.]

Quantity meaning flow and temp? Without a flow meter is there any other method to record this?

Quantity of heat as in Joules in S.I. units or BTU in the old (U.S. customary) system. Since energy delivered is (mass flow rate) X (specific heat of the fluid) X ( temp. diff.), the short ans. is no. It is possible to get an approx. RELATIVE flow rate by comparing temp. changes, in to out, under the same conditions, or the same approx. conditions. That can be of some, but limited use. There are other ways to get approx. flow rates, but those ways will take more instrumentation, and usually some additional information such as input (solar irradiance and collector losses). The best, easiest method is a flow meter and (calibrated - at least to one another) thermometers. For air or other gases, there are hand held turbine meters that can give a good approx. with some practice. For non corrosive liquids such as H2O, the easiest and reasonably accurate device is a rotometer. Decent ones for home use cost ~ $100.