Like most people of my generation, I can vividly remember my first air-conditioned car. Ah, the sheer, decadent luxury! Now, of course, it's so commonplace people only comment if you haven't got it.

I suspect the same thing will happen to boating as air-conditioning becomes the norm and not the exception. It's a necessity for anyone contemplating living aboard for extended periods, particularly if staying in Sydney or parts north of there.

I can almost hear the chorus of horror from the purists. Well, let them sweat all they like, but when it comes to sleeping on a boat in the tropics, nothing beats air-conditioning.

The first question that pops into most people's minds when marine air-conditioning is mentioned is: "What's the difference between a marine system and an ordinary household one?" The answer is "not much," in some ways, and "everything" in others. The best way to explain this anomaly is to first explain how air-conditioners work - including your car's - and then look at the differences dictated to the designer by the marine environment.

THE BASICS
If we look at the sketch below of a basic air-conditioning system, we can see it's composed of four main items, namely the compressor, the condenser, the evaporator and the control device. These four things are connected by pipes in such a manner that an airtight system is formed. All the air is removed from the system and a refrigerant introduced.

Let's start at the discharge outlet of the compressor. The piston has risen up the cylinder bore and compressed the refrigerant to a high pressure, and this high pressure gas flows from the compressor to the condenser. Here the heat is removed from the gas, in this case by the surrounding air.

Remember in last month's article on refrigeration we said that energy travels from high levels to lower ones; well, the same thing happens here. As this heat is removed from refrigerant gas, it cools below its dew point and begins to form droplets of liquid. This is exactly what happens when dew forms on cold surfaces in the morning.

The refrigerant leaves the condenser as a liquid, but still at a high pressure, and this is what ensures it can't begin to boil (evaporate). However, when it comes to the control device, it passes through into the evaporator at a low pressure due to the suction action of the compressor. The control device is just like a throttle and only admits the amount of refrigerant which has been judged ideal by the designer.

Because the pressure has been reduced in the evaporator, the liquid refrigerant now commences to boil. Don't think that a substance has to be hot to boil; these refrigerants will change their state (boil) at temperatures well below freezing. The boiling action of the refrigerant needs energy, and this is supplied from the surrounding air, which, of course, reduces its temperature.

As the refrigerant converts into a gas (steam), it's drawn to the compressor where it's again compressed to a high pressure, high temperature gas, before again flowing to the condenser - and the whole cycle starts once more.

It becomes obvious that the refrigerant is merely the medium that absorbs heat in one part and gets rid of it in another. The condenser is usually sited in an area where the excess heat ejected is not a problem - generally the outdoors - and the evaporator is located in the space we wish to cool.

And that, in a nutshell, is how most air-conditioners operate. The system is simple and reliable. It has been around for years and I can't see anything replacing it in the near future. It uses a fair amount of energy, particularly on boats due to their large areas of glass, so unless you have a generator capable of carrying the load, it can only be used when connected to shore power. We will discuss power requirements later; first we have to decide how much air-conditioning we need.

THERMAL THEORY
Because marine air-conditioning is dominated by US manufacturers, the unit of cooling used is still the BTU (British Thermal Unit). One day the US will see the light and go metric, but until they do I'm quite happy to talk in BTUs. For those who wish to convert, there are 3.4 BTU to a watt, which is the unit used in the metric system.

So how many BTUs do we need to air-condition a typical boat? In this instance, it's always best to take advice from the experts, and for the following information I'm indebted to Gary Kennedy of Seabreeze Industries, which is a leader in this field.

Seabreeze is the agent for Cruiseaire marine air-conditioning - importing these units from the US - and has supplied systems to most of the major boatbuilders.

The company's advice is as follows. Divide the boat into three areas: below decks, mid decks and above deck. Below deck areas are the cabins, where there is generally little sun load from glass area. Measure the length and width of these to arrive at a figure that reflects the area of the cabin. Some averaging will probably have to be done, as it's not often cabins will be nice and square, and the figures are based on the assumption the headroom is around six feet. Because all these calculations are done in the Imperial system, your result will be in square feet.

Naturally if you live in Tasmania, you'll need air-conditioning less than boats moored in the tropics, so for temperate zones allow 60 BTU per square foot, and for tropical zones 90 BTU/sqft.

The mid deck areas are deemed to be the main saloon, etc, and the allowance here is 90 BTU/sqft for temperate climates and 120 BTU/sqft for tropical.

Above deck would be areas such as a glassed-in flybridge. Here, allow 120 BTU/sqft and 150 BTU/sqft respectively.

To make this easier to visualise, let's take a mid-size cruiser around the 35ft mark, with one forward cabin and a main saloon. The flybridge is not enclosed, so we don't have to worry about that. We will suppose that the boat is contemplating a major cruise to the tropics and do our calculations on that basis.

The cabin measures 10x8ft and the only sun load is a small hatch in the deckhead. Based on that, we come to 80sqft and, allowing 90 BTU/sqft, this gives a figure of 7,200 BTU. The main saloon comes in at 140sqft and this will need 16,800BTU for the tropical regions.

The next thing to consider is the electrical load this will impose. From the specifications, a 7,200 BTU unit will need about 1.5kW of power and the 16,800 BTU unit around 2kW. That's a total air-conditioning load of 3.5kW, but allow 4kW to be on the safe side, as there are pumps and fans to operate as well.

Now, because these are mainly motor loads, they will be highly inductive and that means they will have very high starting currents. In my opinion, the smallest 240V generator that could happily run these items without any starting problems would be 6kVA. This would allow the genset to have some reserve for other items when the air-conditioning is operating.

The other thing to notice here is that 4kW is 16 amps at 240V supply, and that is generally the maximum you can expect from the outlet socket installed at the average marina berth. One day the marina industry will catch up with the enormous power requirements of the average powerboat, but from my observations they are still lagging behind. Remember that all we're talking about is a 35ft cruiser!

If you're plugged into shore power with both of these units running, there will be nothing left to operate anything else - no toasters, no hair dryers, no TV, nothing. The way most boats overcome this is to drop out some of the load, eg, one of the units. But this can be a nuisance and can require many trips to the shore circuit-breaker on the pole at the marina.

The other alternative is to sit in the marina berth with the generator running, but I can guarantee you will have a zero popularity rating if you have close neighbours!

THE TOOLS FOR THE JOB
So, having decided to air-condition the boat, the next thing is selecting equipment for the task. Regardless of the manufacturer, marine air-conditioners basically come in two styles: self-contained or remote. The self-contained units are a complete air-conditioning package on their own, similar to the window units made for domestic use. The major difference between them is that the marine unit has a watercooled condenser and the domestic unit has an aircooled condenser.

The use of water to cool the refrigerant gas results in a quieter and more compact piece of machinery. If using one of these for the forward cabin, they are generally located under the bunk. The plumbing for the cooling water generally runs to it from the engine room.

The sketch at top left on the previous page shows a typical installation. Note that the unit's cooling air is ducted up the inside of a hanging locker and discharged as high as possible into the cabin. This is because cold air is more dense and sinks, creating good air circulation and providing the most even cooling possible. The air returns to the evaporator through a return air grille cut into the base of the bunk.

These self-contained systems work well and have the advantage that they can be installed by anyone with reasonable skills. The downside, if there is any, is that the machinery noise is right there with you in the cabin. I have never been too bothered with this, as a good installation with lots of soundproofing means the sound can be reduced to almost nothing. In my experience, it's the outlet air grille that makes the noise.

The other alternative is the remote. As the name suggests, the machinery part of the air-conditioner is split from the cooling head and located in a remote part of the vessel; thus the noise problem is overcome and the cooling head, being much smaller than the total unit, is much easier to fit if the area is confined. These systems will generally need to be installed and commissioned by a qualified air-conditioning mechanic, as copper tubes must be run to and from the condensing unit and the evaporator head.

The system that you ultimately install will depend very much on your boat, as room to retrofit these items is generally the deciding factor.

PUMPED UP
The plumbing for the cooling of the refrigerant gases is normally achieved using a small 240V centrifugal pump. Centrifugal pumps don't have the ability to provide suction - think of them as an enclosed water wheel that moves water along a pipe and must therefore be mounted below the waterline. They provide good lift and are extremely reliable and quiet. However, they are annoying if they get air into the system, as they are not self-bleeding.

For boats that can plane, this can lead to irritation beyond measure, as in some instances air will roll along the underside of the hull when the boat is under way, and this air can find its way into the inlet pipe of the pump. When you get back to the marina and decide air-conditioning would be nice, the air now in the pump forms a blockage that won't allow the water to pass.

In this case, there is no alternative but to get down in the engine room and bleed the air from the pump. This can be a bugger of a job to say the least. Refrigeration pumps suffer from the same problem.

To ensure there's as little chance of this happening as possible, make sure the piping from the skin fitting to the pump rises continually, with no kinks or dips, and then the same to the unit. This will generally fix the problem, although some pumps have a mind of their own. The alternative is to fit a self-priming pump, but these are generally very noisy and expensive.

One pump is capable of serving more than one unit, and if this is done then the pump will have to be switched through relays. Most manufacturers can supply these with the rest of the equipment.

STOP THE ROT
Another thing to be particular about in the installation of any air-conditioner is the condensate water plumbing. This is the water that condenses on the evaporator, especially when the humidity is high. The air-conditioner on my boat makes over a bucket of freshwater a night, which can cause a rot problem if not disposed of properly. I simply piped mine to the bilge, but I can understand some may be reluctant to do this. If that is the case, a small sump with an automatic float switch and pump will have to be installed.

One option often offered is reverse cycle heating. Without getting too technical, the system is valved so it can operate in reverse - the cooling heads become the condenser and vice versa. This provides abundant heating throughout the boat, but it's not an option I would bother with, as small fan heaters are cheap and do the job without all that machinery thumping away. Cool-only air-conditioning is a lot cheaper.

COUNTING THE COST
Finally, what does all this cost? The short answer is heaps! To fully air-condition the average cruiser, you'd better have deep pockets.

There are a few alternatives. Air-conditioning only the sleeping quarters is one option. Another is to do what I did: have one large unit for the main saloon. I made a changeover plenum in the ducting, so I could direct the air to the cabin for sleeping. This means I can cool the saloon or the cabin, but not both. I find this an acceptable compromise.

As far as other solutions, I have seen people convert domestic air-conditioners for use on boats with varying degrees of success. If the budget is tight, this may be worth looking into.

Well, that's about it for air-conditioning. It's wonderful to have on a hot and sticky night. For day-to-day use, I could probably live without it. I could probably live without a lot of things on my boat, but I've no intention of trying.

Next month we'll return for a second look at refrigeration, as promised in last month's instalment.