A/C cooling capacity question...

[justinc]

Hoping some of you automotive A/C experts can help here.

I have been rethinking the A/C requirements for the 110, and have come across a dilemma regarding interior headroom, and now thinking a 'Red Dot R-6830' unit (http://www.rdac.com/Pages/product_pages/units_pages/6830_unit.html), mounted between front seats may be a better alternative. It is rated at 17,000BTU/Hr, and 260CFM flow rate at max fan setting. Is this going to be sufficient to cool a 110 interior? Or do I need to fit 2 of them? (Back to back, between the front seats?)

Any help/ comparisons with factory air performance etc would help immensely.

JC

[Blknight.aus]

@5KW of cooling it should be about on the money the problem is going to be getting the air to circulate to where you want it.

[blitz]

bloody BTU's archaic terminology ;)This is 1 BTU = 0.0002931 KW

Apart from Blknight, lada, myself and a few just plain smart buggers who are in the refrigeration industry and old enough to remember it no one in the industry uses it but sellers still do, we dropped it when metric came in back in umm 66? so long ago I cant remember.

Now that I have had my rant :D:D:D

as blknight said 5kw should be plenty, (I will conceed he is better at auto air than me hands down)

[rick130]

Never done a heat load on a car, where would you start other than 72w sensible per person :confused:

If it was a coolroom or domestic/commercial a/c system no worries ;)

and yes, I need to drag the conversion book out if I come across BTU's or Tons of refrigerant.

[scarry]

The reason all that BTU crap keeps appearing is because the Chinese stuff is rated in it.Horse Power is also useless,but some people,including suppliers keep using it:mad:

Anyway,back to heat loads in vehicles,sorry,have no idea:confused:

In fact i think they have changed the regs so i am not supposed to work on auto air anymore,not that i would want to ......

[justinc]

...And BTU's are a USA thang, Red Dot are a US based company.

Doing my head in a bit, i did go through it wayyy back when, but can't remember any of it now:(

I was also interested if the flow rate would be enough, some slightly bigger units advertised by them have 400CFM+!

Anyway, keep the replies coming, it'll help immensely to find the right unit, without just reading through a catalogue:(

JC

[blitz]

I guess one way of making a comparison Justin is to get the size of the compressor that it comes out with, then you have a starting point; Obviously the evaporator needs to be sized the same so that it matches refrigerant flow rates.

Air flow? due to the amount of glass area and volume I would suggest the more the better as a rule of thumb, you can always turn the fan down but if you dont have enough when it's flat out then you have aproblem.

Sorry I cant be more accurate than that, all i can say is, in a vehicle over sized is better than undersized for the above reason (within reason :cool:)

Oi do you really need it in Tassy:angel: :D

[blitz]

Never done a heat load on a car, where would you start other than 72w sensible per person :confused:

If it was a coolroom or domestic/commercial a/c system no worries ;)

and yes, I need to drag the conversion book out if I come across BTU's or Tons of refrigerant.

When I was doing cars, trucks and earth moving equipment - over 20 years ago in the Territory (and apart from my landy it's been that long since I played with them) we used to cram in the biggest unit we could that wouldn't stall the engine, never had a complaint :D

[slug_burner]

I think it is 1120 watts per square metre of surface area that the sun contributes to heat load. Then there is the issue of external air temp and humidity as in habital spaces you require a certain amount of fresh air. In vehicles I am not sure what percentage fresh air is brought into the vehicle when it is on recycling the interior air only. Five kW of cooling is a fair size aircon but I expect you would need that or better to maintain the ambient temp inside the vehicle due to the poor insulation.

I think that most vehicle aircon systems rely on the cold air ducts pointing at the body and therefore getting a greater degree of comfort to those that have control of the vents.

[scarry]

...And BTU's are a USA thang, Red Dot are a US based company.


JC

My mistake,yes they still do use it in the USA,But many of the Chinese split A/C's are rated in BTU's.

Could you find out what ,say,a D2 A/C system is rated at,therefore giving you a reference to start on.The D2 seems to have a very good A/C system,although it is smaller than a Defender.

[Blknight.aus]

the rough numbers I use are

1KW per M2 of glass

.2kw per seat

Btu is british as in British Thermal unit. As in enough energy to heat one pound of water one degree farenheight

for some really really rough conversions.

a BTU
is about equal to a Kilojoule
is worth about 780 ftLbf

[slug_burner]

What about the non glass surfaces? They still capture those rays otherwise sealed windowless boxes would not get to 70C when sitting in the sun.

[isuzurover]

Gerry (1103.9D???) has one of these: Red Dot's Product Line (http://www.rdac.com/Pages/product_pages/units_pages/6100_unit.html) on his county - note it is 16000 Btus. When I last spoke to him he was happy, but would have preferred the model with an inbuilt heater.

I asked Chuck from Haultech Engineering about AC, and he recommended the 6100. His words were something like - it is small capacity, but the cold air blows on your head where you want it.

Haultech design and build truck AC systems for a living.

[rick130]

What about the non glass surfaces? They still capture those rays otherwise sealed windowless boxes would not get to 70C when sitting in the sun.

Absolutely, but it's just a rule of thumb.

When I calculate a heat load for a house, I use different numbers for the windows facing North, North east, North west, etc, etc, as well as using different figures for different times of the day (10a.m and 4 p.m. in my system)
Then I use different multiplication factors for cavity brick, brick veneer, weatherboard, etc. suspended floor vs enclosed crawl space vs slab.

With a car the sun can be coming from any angle, then you have the conducted heat through the firewall and floor, respiration load of the passengers (as I said above, I use 72w/person when they are sitting) and it becomes hard to calculate.
You'd almost have to sit the vehicle in a lab and measure, but over the years 'rules of thumb' have been established which can shorthand/shortcut the process and get pretty bloody close to the right result.

With domestic and commercial air everyone else up and down the valley here just use 130-160w/m2 (floor area) and their systems most nearly always work OK.
Maybe I'm the dumb one :confused:

[justinc]

Gerry (1103.9D???) has one of these: Red Dot's Product Line (http://www.rdac.com/Pages/product_pages/units_pages/6100_unit.html) on his county - note it is 16000 Btus. When I last spoke to him he was happy, but would have preferred the model with an inbuilt heater.

I asked Chuck from Haultech Engineering about AC, and he recommended the 6100. His words were something like - it is small capacity, but the cold air blows on your head where you want it.

Haultech design and build truck AC systems for a living.

Thanks Ben, yes Gerry is very happy with his unit, I was trying to get away from external roof mounted ones, for clearance issues IE putting the 110 on the train trip one day...:D, or else I could fit air suspension and smaller diameter tyres for tht trip...:o
I still have a rear evap unit from an HJ61 Sahara which is ducted up the side of the vehicle and across the top of the rear seat passengers, and by all accounts works very well, but haven't got any specs etc to compare.

Anyway, need to think a bit more still....:(

Thanks everyone for input:)


JC

[Blknight.aus]

What about the non glass surfaces? They still capture those rays otherwise sealed windowless boxes would not get to 70C when sitting in the sun.

its covered by the rough rule that the glass area of a vehicle is only ever roughly 1/3 of the cabins surface area. and that .2kw per seat is more than enough to offset what a person at rest can generate.

Things that arent considered in the formula that I use as my rough rule of thumb are radiant heat from the road, engine and transmission heat transfer. (which only really come into account if your idling and stationary or have the thing loaded up to the nines)

Whats more important than the cooling capacity of the Evaporator is the capacity of the condenser and the pumps ability to shift the refirgerant.

the TD5 deefer setup is quite good in that respect theres a decent overhead on the evaporator and the pump.

very very roughly every 150-200grams of refrigerant will give you one HP of cooling from the evaporator inside the cab and draw 1KW from the crank. (its variable becuase of the varying effeciencies of different pump styles and condensor/reciever drier configurations.

IMHO the most important thing you can do when you setup your aircon is fit a moisture indicating Liquid level gauge in the line after the reciever drier.

[justinc]

its covered by the rough rule that the glass area of a vehicle is only ever roughly 1/3 of the cabins surface area. and that .2kw per seat is more than enough to offset what a person at rest can generate.

Things that arent considered in the formula that I use as my rough rule of thumb are radiant heat from the road, engine and transmission heat transfer. (which only really come into account if your idling and stationary or have the thing loaded up to the nines)

Whats more important than the cooling capacity of the Evaporator is the capacity of the condenser and the pumps ability to shift the refirgerant.

the TD5 deefer setup is quite good in that respect theres a decent overhead on the evaporator and the pump.

very very roughly every 150-200grams of refrigerant will give you one HP of cooling from the evaporator inside the cab and draw 1KW from the crank. (its variable becuase of the varying effeciencies of different pump styles and condensor/reciever drier configurations.

IMHO the most important thing you can do when you setup your aircon is fit a moisture indicating Liquid level gauge in the line after the reciever drier.

Thanks Dave, thats why i'm using a D1 (For R134a gas) condenser with the twin fans, and a new Sanden compressor. The D1 has a very good airconditioner in my experience, so thinking this may be easily enough.

JC

[Dougal]

Never done a heat load on a car, where would you start other than 72w sensible per person :confused:

If it was a coolroom or domestic/commercial a/c system no worries ;)

and yes, I need to drag the conversion book out if I come across BTU's or Tons of refrigerant.

I'd factor in 1000w for every m^2 of glass exposed to the sun. How much insulation you've got in the roof, firewall and doors will be a big factor too.

*edit* Should have read page 2 before posting.

Do these units work in reverse as heat-pumps? Might not be as big an issue in Aus, but if I could then I would get a reversing one here for NZ.

[rick130]

[snip]
and that .2kw per seat is more than enough to offset what a person at rest can generate
[snip]

Whats more important than the cooling capacity of the Evaporator is the capacity of the condenser and the pumps ability to shift the refirgerant.

[snip]

As I mentioned above, we use 72w sensible/person at rest, 80.6w light activity, 89.5w heavy activity (and I'll leave that one up to your imaginations :angel: ) in domestic/commercial a/c calcs.


The condenser capacity is the compressors refrigeration capacity at it's design suction temp. plus the heat of compression (roughly 20% on top of your evap load) to deliver a particular saturated liquid temp. plus a fudge factor to allow for dirt/dust/crap.

This can be calculated with a pressure/enthalpy chart
Actually you can derive the total system pressure/temps at any particular point with a PE chart.

I tend to size commercial/industrial refrig condensers @ 12K TD, (condensing temp is 12K above ambient temp)
In the old days 15K TD was used but I'm guessing a car a/c system would be higher again, just through size limitations.
I have an old Heatcraft car a/c catalogue here but can't find any design data ATM.

FWIW the biggest Sanden universal off the shelf condenser that will fit a Tdi Defender is the one supplied in the old AMC system, as far as my searching has found.
Sanden # MFC-2068AE according to my old catalogue.

[rick130]

[snip]
Do these units work in reverse as heat-pumps? Might not be as big an issue in Aus, but if I could then I would get a reversing one here for NZ.

Size limitations might restrict you installing a reversing valve in one, but I can't see why it can't be done with a 12v coil on the valve.

IIRC (I was only a kid) Dad built the first reverse cycle Daikin system in 1975 by engineering and installing a reversing valve in one of their 5HP ducted systems.

This was then packaged and sent back to Japan by the then importer (Clark Daikin) for evaluation, as prior to this the Japanese just used electric element reheat, yet all the Aussie manufacturers had been reverse cycle heating for years.

[justinc]

...See http://www.aulro.com/afvb/technical-chatter/121797-now-i-know-i-married-right-girl.html#post1417076 for a continuation of the A/C idea...

JC