Air Conditioning on Enclosed Fly Bridge

Hi,

1 watt is 3.41 Btu per hour so your 62,000 Btu will be approx 18 kW, just a bit too much for your 17 kW Genset.

As to the ability of the 24,000 Btu machine to cool the enclosed Flybridge it will depend a lot on where it is located, how well the hartop if fitted is insulated, the volume of the space, what the sides are made of etc.

Hi,

If there is some type of hard foam insulation that you could spray the underside of the hardtop with this will help somewhat.

I would guess that the 24K Unit might make some difference but not enough to allow you to shut all enclosures and chill on a hot day.

You will definitely be maxing out the capabilities of that genset but with a little power management can make it happen Unleashed/Wangberg. You may have to shut down one or two of the stateroom units and possibly other large consumers (icemaker, watermaker, washer/dryer) but if you'e on the bridge underway that won't be to hard to stomach. I'd also recommend installing a soft start with that unit which will greatly reduce the compressor surge current. Cruisaire offers two types and are well worth the investment especially if you're close on demand for KW's. As others have already said, it won't turn the bridge into an icebox but it does help a lot on those hot, muggy days to have a cool blast of "angels breath" blowing at you. Pay careful attention to vent location so you can direct it where you and your guests will be standing/sitting.

I'm not a hvac guy, but the subject of A/C on boats (homes as well) is always very interesting to me. Size of system, ways to enhance it, etc.

That is great info that K1W1 provided. In addition to what he mentioned you may also want to look at Cruisair's website. They have a section on general guidelines on how to determine the size of A/C needed.

Here is a little snippet that I c&p'd from their site to give you an idea of what they suggest. You'll have to go to their site for a full reading including a chart to help with figuring heat load.

1 - Required Capacity
Divide your boat into three basic load areas:
(1) Below Deck – cabins where the hull slopes inward toward the keel and there are minimal port lights and hatches.
(2) Mid Deck – areas on the main deck with small or shaded windows.
(3) Above Deck – areas with large glass surfaces and direct sunlight.

Measure the length and width of each room to be conditioned to determine the square footage. It is assumed that your boat has average headroom of about 6-1/2 ft. (2m) and you have an average amount of furniture. If one end of the compartment is narrower than the other, take your measurement in the middle. Multiply the length by width to get the area of each.

Determine which load factors to use and multiply the area of each cabin by the load factor to determine the required air conditioner capacity.

Back of the envelope calculations call for an energy consumption around 30 percent of the cooling load expressed in kW for a well matched system.

For a system like the OP described, which is about as bad as it gets, the electrical load in kW can easily exceed the heat load expressed in kW.

If it runs constantly, as it probably will, it may consume up to 5kW.

The hard part now is to calculate the heat transfer rate into the space and decide just how much you want vs what you need. There are plenty of tables and calculators available to help answer that question.

Actually I have installed several AC units on flybridges and cockpit mezzanines and they do make a noticable difference especially on hot calm days. If it's a three sided enclosure it will have less benefit than a four sided one but it still will help take the edge off. A directed stream of cooler air blowing in your face makes a lot of difference some days!

Most marine HVAC units will only reduce (or raise) the inlet air temp by an average of ten degrees so obviously if it is constantly trying to condition new air the benefit will be less than if it is recycling already cooled-down air. You may not need togo with a 24KBTU unit on that bridge either, all the ones I've done have been either the older 16K or the newer and much more powerful 18K Turbo units which flow much more air than their predecessors and consume less power.

Here's what is wrong.......While I agree 1 watt=3.412 BTU/hour you can't calculate the electrical load ,64,000 Btu/hour backwards unless you know the systems COP (coefficient of performance). I believe K1W1 divided 3.41 into 62,000 Btu/hour to get~18kw. To do that you have have to assume the COP is 1. All AC systems and heat pumps have COP's higher than 1. So lets say the COP is 4 (maybe only 2 or 3)that means for every watt used in the "heat of compression"(technical term)(laymans term;the work the compressor does.) you can move 4 x 3.412 Btu/hour of heat which equals 13.648 btu's/hour for every watt of electricty used.

There is 746 watts in 1 horsepower.
There is also a very old & very rough rule of thumb that it takes ~ 1 horsepower to move 1 ton(12,000BTU/hour) of cooling plus wattage for the blowers/fans. so 24,000 BTU/hour AC unit would use ~ 2 x 746 =~1492 watts of electricity. So add blowers/fans and your around ~1800 to 2000 watts of power, You can covert the wattage to amps if you know the voltage.

You should look up the amperage or wattage on the systems data plate.

Correct.

K1W1 would be right if you were talking about a heater where you convert the energy in electricity to energy in heat.

Air conditioners on the other hand are strange. They are a "heat pump" where you are pumping the heat from one fluid to another.

You are not creating heat, but instead as Wdrzal points out merely "moving" it.

What you are doing is taking the heat out of the inside air and putting it instead into the seawater. The air gets colder, but the seawater returning to the ocean from the heat exchanger is correspondingly hotter.

This process however isn't free. Second Law of Thermodynamics says that heat will try to flow from a hot object to cold object, and to make it go in reverse (like an airconditioner does) you have to perform work on it (which is what your a/c compressor is doing).

The key concept is called Entropy (or Randomness). All systems tend to a state of maximum randomness, and to make them more ordered (less random) you have to do work. Taking warm air and warm seawater is natural state of randomness. To convert this to cold air and hot seawater is a more structured (ordered) state. This takes work (provided in this case by the electricity).

So an analogy ... my son's bedroom. At 7:00am it is neat and tidy. Happy mother. By 10:00am is spread with a uniform layer of lego pieces etc. happy son. To go back to a "happy mother" by 7:00pm, someone has to do work. That room is never going from maximum randomness to nice and tidy, without the work being done. Air-conditioners are doing exactly this sort of work. All the lego is still there, just it is back in the boxes ....

So, the good news here is that you don't need to move much seawater to cool a large volume of air.

Even better news is that is why reverse-cycle airconditioners are so much more efficient than a heater. Typically you can get 3x as much heating effect by effectively cooling the seawater and warming the air, than just burning the electricity itself to generate heat.

This multiplier is the COP as referred by Wdrzal above.

It might be different in the boating world, and I am a noob there , but in the land-locked world of residential construction, COP is generally used to refer to heating ability and SEER (or EER) refers to cooling ability. Generally because most homes don't use the same heat pump for both heating and cooling unless they are running a geo-thermal system.

Anyway, the point is that modern air source A/C units are required (federal law) to have a EER of 13+ (e.i 381% efficiency) in building construction and that is what manufacturers are now using as a baseline. 14-15 is becoming more standard and you can get them up to 18 although it is expensive. For some time most tech advancement was related to air source heat pumps as the market is bigger, so they were getting very comparable to water-source heat pumps in terms of efficiency. No I am seeing open loop water source heat pumps with COP = 5.1 which would be an EER of 31 which is unheard of in the air source world.

EER/3.413 = COP

So a 14 EER A/C unit would have a COP of 4.1 so a 1kW gen should power a 13,600 Btu A/C unit. So Pascal's observation that he can run 100K Btu a/c on a 18kW genset is true because with a modern unit he should be able to get almost 250K Btu if that was the only thing running on his boat.

My biggest question on these boat systems is their source. Do they typically use ocean/lake water for heat transfer, or do they use air? I would be astonished if they used air when there is such an abundant and stable supply of water available that would dramatically improve the efficiency of the system, although I wouldn't think of using an open loop system in salt water.

Yes, they use an open loop system in saltwater......there is a pump that sucks the water in with a sea-strainer to catch larger debris, it then goes through all of the units to cool off the heat transfer (usually with double walled piping, then the water discharges out of the side of the boat......

So IMO this ties in with the thread "are SF's dead". Who the heck worried about Ac up there?

Wow, no worry about the corrosion caused by the salt water? Must be full stainless or something? In the systems I've seen, oxygen alone results in corrosion, let alone oxygen and salt.

Everything is bonded. Corrosion where the coils fail before the unit itself has a major failure is on the rare side. It happens once in a while, but usually I've seen units last anywhere from 12-20 years and the compressor usually dies first. You would get some corrosion from the oxygen disolved in the salt water, but that type of corossion is a lot less than parts that get salt exposure and are exposed to the atmosphere. Electrolysis tends to be a bigger issue in metals with saltwater (and/or freshwater) running through them if they're not bonded correctly. Usually the coils are made out of copper or bronze.

Stainless tends to not do as well when you have water running constantly through it. Stainless Sea-strainer baskets tend to get eaten up in the sea-strainer side of the a/c system about once a year or sometimes more often.

Getting growth in the lines and units from barnacles and algae and stuff in the saltwater (and FW) can be a problem to, but the lines and units can be cleaned by recirculating various flushing products through them.

I do find your knowledge of residential hvac systems to be amazing and the conversions that you posted and have learned from them.