chilled water air conditioning

[beau]

I plan to install a 4hp Honda commercial four stroke motor driving a Sanden (car type air con compressor) which puts out 12,000btu's/hour this will cool/freeze a 25 Gallon well insulated tank filled with salt water. I plan to run it for 2-3 hours a day to store the 24,000 - 36,000 btu's of cooling capacity.
The chilled water will then be circulated (small pump) to a fan forced evaporator. This should give me 5,000 btu per hour of air conditioning.

Are my sums right, have i missed something, will this work?

The alternatives are to run the Honda motor all night, Noisy. and the 12DC electical requirements for air con are just too high.
Any comments would be appreciated.
regards,
Beau

[Pblais]

One BTU is one cubic centimeter OF water 1 lowered degree Celsius times 12,000. You won't be freezing 25 gallons of salt water in the summer at this rate. 1,000 CC's = 1 Liter. Liters to gallons I'll leave to your own calculator.

The alternatives are to run the Honda motor all night, Noisy. and the 12DC electrical requirements for air con are just too high.

That math is correct. You can't take a short cut with the math. It's a lot of power and a way more than that at 12 volts DC. Just imagine running the car A/C for 3 hours. How well do you think it would work and how long would it last on the boat? You can convert back and forth but it's still the same BTU's. You are not just cooling the air inside the boat there is the boat itself that is as warm as the air that also needs to cool so you can feel cool or actually stay cool.

[Talbot]

I suspect that your 4hp would not even manage to turn the compressor anyway. I thought of going this route, but gave it up whrn I discovered that the compressor in my car was pulling 8 hp.

[beau]

A car type air con compressor uses 1.5 -2 hp.

Chilled water air conditioners are used commercially, and they seem to work well.
I don't think pblais calculations are correct, 12,000 btu's in 12,000 out, if not, where is the loss going?
and what is wrong wilth running a compressor for three hours.(yes I know they are designed to cut in and out, but have you ever been out in your car on a HOT day and needed every bit of cooling, the compressor is ON full time.

I can run the small 4hp Honda full time when using the air con, say at half throttle. and It is possible to build a very effective sound absorbsion box using a small 12volt fan to supply air through a series of baffles etc at both ends. These systems are very popular in the camping market

[jim lee]

How about a 30' hose with a weight at the end. Pump cool seawater from 30 feet down around a cooling circuit with a little 1A electric pump. If 30' don't do it.. maybe 50' would?

Or is the water even that far down still to warm to work?

-jim lee

[hellosailor]

"A car type air con compressor uses 1.5 -2 hp. "
Not that I don't believe this, but most cars react to the AC being engaged as if they're pushing a LOT more load than that. What compressor is this hp spec supposed to be for?

I'm thinking you will wind up with a perpetual motion machine. No one has ever found a "cheap" and "simple" way to provide air conditioning.

[Pblais]

I did more reseacrh and here is the correct number to use. I am sorry my first response was incorrect.

1 BTU is the energy required to raise one pound of water 1 degree F. That number will vary by up to 0.5% as water does not hold the same amount of energy at all temperatures.

2500 BTU's is equal to one horsepower.

You'll need a 100% efficient process to make even these numbers work.

[ssullivan]

A used off the shelf air conditioning unit would be vastly more efficient, cost the same or less, and save you countless hours of work. Why not just buy one? It would be a waste to have a dedicated engine for AC (think oil changes, etc...). Get a generator (used again if you're on a budget) and an electric AC if it's that important. I have 2 AC units and I have never used them.

[GordMay]

Density is mass per unit volume, where pure* water is determined to be 1.0, and ice (at 32 deg. F) is 0.92 (which is why ice floats in water).
Hence, freezing a gallon of water increases it’s volume by about 9% (1.0 ÷ 0.92 = 1.087).
Your 25 Gallon tank could hold about 23 gallons of frozen water (ice).
Since water weighs 8.33 Pounds per Gallon, your tank will hold 191.59 Pounds of Ice.
Since the latent heat of fusion of water/ice is 144 BTU per pound, your 192 Pounds of ice will absorb about 27,508 BTU’s of heat in melting.
If we (arbitrarily) assume a 24 hour cycle, the ice will provide about 1150 BTUh of cooling.
This is approximately 1/4 the size of the smallest conventional air-conditioners.
The “rule of thumb” recommendation for sizing marine air-conditioning is about ± 14 BTUh per cubic foot of area.

*Seawater & sea-ice has a density of about 1.03.

1 HP = 2545 BTUh

[beau]

I would like to thank everyone for helping.

Gordmay is right with his calculations (I think) in that the 25gallon tank will hold 27,000 btu 's . What I am planning is to pump the ice water to the evaporator at a high rate at first, probably have the honda motor still running to pull down the temperate (after dark, in a very well insulated cabin) Then when ready for bed turn down the flow to approx 3,000 btu's per hour until the cooling runs out. It gets cooler late at night anyway.

I live in Tropical Australia it is difficult to sleep in high humidity in a boat.

Many Campers here have made Air conditioners by using a car air con " Sanden" and running it with a 2-3 hp honda motor runninng at 2,500- 3,000 rpm They use an electic radiator fan to cool the evaporator. The latest idea is to put them into a soundabsorbing box. The 12volt ex car radiator fan pushing air through the box, though a series of baffles. Standing beside one of these you can only just hear it running.

I must admit I am puzzled by the reference (twice) to 1 hp = 2,500 btu's
I don't know what to make of that figure.
Car airconditioners "Sanden" put out 12,000 btu's per hour and we run them with 2.5 hp Honda motors.
even Domestic air conditioners of 1.5 hp run 20,000 btu's/hour.

[hellosailor]

"a 25 Gallon" So, roughly 200lbs. of water, or 200 BTUs per degree F change in that?

And most car AC units are designed to cool the outside medium to about 50F so a paltry 200BTUs and dropping the water from, let's say 95F to 50F, 45F difference...would only need 9000BTUs to chill the tank, ignoring the losses.

But I think there are some bigger issues than the math. An AC system needs both condensor and evaporator units. Where's the condensor cpil going to go? What condensor is going to handle the load? In efficient dedicated marine AC systems, that's going to be a cupronickel or monel assembly cooled by raw water to gain efficiency, instead of requiring massive forced airflow. (The condensor has to exceed the transfer rate of the evaporator, or the system runs out of liquified coolant and stops cooling, right?)
And similarly, the evaporator, which is going to be immersed in a water tank, will need to be cupronickel or monel. WAY more expensive that then copper and aluminum used in "air" units.

Dunno, Beau. To paraphrase Ricky Ricardo, "Lucy, you got some 'splainin' to do!" In the sense of, have you even considered those issues?

The one fast dyno test I found:
Your car's hidden Turbo Boost Button. | Tech Talk Blog & Discussion at Automobile Magazine
indicates ELEVEN HP to run a compressor, three times what your motor can put out at full load. Which is more in line with what I'd traditionally been told.

[beau]

I am sorry to correct "hello sailor" and "talbot" but a car type airconditioner compressor only uses 1-2 hp max.
I called the manufacturer of Sanden which probably makes 80% of all car compressors.
Also read Steve Dashew (setsail.com) who talks about using chilled water airconditioning on his boats

[hellosailor]

"Sanden which probably makes 80% of all car compressors."

That's interesting. Has Detroit quietly switched to Sanden? Or are they the prime makers of the compressors sold under the Denso brand on Japanese cars? What are Sanden compressors used on? I've only seen their name referenced in the second tier (non-OEM) market, never actually in a vehicle with OEM equipment. Their web site boasts only about being the largest *aftermarket* independent supplier, which makes me think they are tiny players compared to the ones affiliated with carmakers. Not that that makes their equipment or data bad--just to set it straight who they are.

And do they recommend their use for chilling water? (I'd expect the heat transfer dynamics to be a bit different than air transfer systems.)

From home air conditioners using 120VAC, I see that 2000 watts (3hp) buys about 7000 btus, but I've never seen anything printed by a prime source that indicates their car AC compressors use so little power. My understanding was that car systems are way less efficient because of the different compressor design.

I'm not sure how to divine the normal operating point from Sanden's data plots, but they do show pwoer consumptions from 2-8hp on the performance charts on their web site, i.e. :

http://www.sanden.com/support/pdf/SD7_brake_curve.pdf

which makes me think that 2hp is a bare minimum for a unit that's running in a very small car on a very cold day.

The only Dashew/setsail article I found was:
SetSail.com - the serious cruising sailor's website
Which refers to running multiple electric AC units on their boat and makes no mention of anything unconventional about them. What article did you find about chilled water?

I've known chilled-water systems in industrial installations, but never on a small craft. Among other things, that 200-pound water tank you are planning to use is a significant weight. Then you have to circulate the water from that tank to, basically, baseboard heating units (cooling units) with separate forced air on each. Lots more cupronickel, weight, and power. Have you gone beyond the "cool idea" stage to figure out the weights and expenses of all that incidental plumbing?

The Dashews of course are on what most of us would call a damned big yacht...Using 54,000 BTUs and a 5kW power drain, and saying up front that he'd need twice that much power without their extensive awnings.

[beau]

I stand by my figures of 1- 2 hp to drive any common rotary car airconditioner compressor.
I have seen them running on 2.5 hp Honda motors. And they are regularly used on boats as a refrigeration compressor by changing the gas.

Sanden also make the Denso range of compressors or is it the other way around but it doesn't really matter.

I am sorry, your figures don't add up.
What is all this about 1hp=2,500 btu's.
Single phase 110 volt is flat out supporting 1.5 hp and yet most domestic air conditioners are running 20,000 btu's
Please explain?
Where am I getting it wrong?

Don't try an insult me or try an ridicule me just give me accurate information or do not give it at all.

The article on Chilled water air con is in Steve Dashew's Cruising encyclopedia. He does not discuss the storage of the iced water but he does support the idea of Chilled water air conditioning.

[Pblais]

The Dashews of course are on what most of us would call a damned big yacht...Using 54,000 BTUs and a 5kW power drain, and saying up front that he'd need twice that much power without their extensive awnings.

I'ld say that was actually a very efficient estimate unless they like it really warm. 1000 BTU/h is approximately 293 Watt. That means 15,822 watts plus what ever it takes to account for the fact the system is not 100% efficient.

Chilled water really is a nice system if you need to cool the condo at the beach. Very quiet system and low maintenance. Many buildings just sell you chilled water by the gallon. It can be very economical when spread out amoung 100 units. It's just not something you store up for later. It works best if you just leave it on all the time.