I know just enough about air conditioning to get myself into trouble at cocktail parties: so this is not a "how to do it" article but the story of how I went about cooling the cockpit of my '40.
…because it needed cooling. I didn't build a/c in initially partly because of time and money and partly because I thought "air" was for cissys. 3,500 miles made it clear that this was a mistake.
The Way it Works
Air conditioning is simple in principle. Essentially you have a loop of pipe containing a refrigerant connecting a pump sandwiched between what are in effect two radiators. The radiator upstream of the pump allows liquid refrigerant to expand and turn into gas, which cools the radiator. This is where you get your cold from. The pump compresses the cool low pressure gas back into a hot high pressure gas. The second radiator cools the hot gas, it turns back into a liquid and round you go (Fig 1):
Fig 1: Basic a/c cycle
The first "radiator" (on the left in the picture) is called the "evaporator" and is a bit like a heater matrix. It sits in the passenger compartment and has a fan blowing though it. The other "radiator" is referred to as a "condenser" and is akin to an ordinary coolant radiator. It sits outside the passenger compartment somewhere and again usually has a fan (which may be shared with the ordinary coolant radiator fan)
The pump is normally belt driven off the engine and is very roughly the same sort of size and shape as a power steering pump, small starter motor or old-fashioned dynamo.
Being belt-driven the air conditioning pump pulley runs any time the engine is rotating. The pulley typically incorporates a magnetic clutch. Fundamentally, you turn the air conditioning on by energising this magnetic clutch. The pump starts pumping, the condenser warms up, and the evaporator cools down, causing a delicious coolness to invade the passenger compartment. Typically, evaporator and condenser fans are also started when the clutch on the pump is energised.
It is just about that simple, although there are a couple of other bits and pieces. Firstly, you need to have some extra refrigerant sloshing around the system, much as you might have a header tank in your cooling system. There are two alternative arrangements here.
1 The system in my car uses a "Receiver Dryer" which is located between the condenser and the evaporator, plus an expansion valve built into the evaporator.
2 The alternative style is an "Accumulator" between evaporator and pump, plus an "orifice tube" between the condenser and the evaporator
Secondly you have a thermostat which de-energises the pump clutch when it reckons the evaporator core is cold enough. As you drive along the thermostat is causing the pump to drop in and out to keep the temperature in the chosen range.
Thirdly there will be a pressure switch. If the condenser cannot cope, the pressure in the system builds up. The pressure switch may be used to notch up the condenser fan as the pressure rises, and if pressure continues to build up it will eventually de-energise the pump clutch until the pressure comes down. We are talking quite high pressures here - maybe a couple of hundred p.s.i. or more. Putting it all together, it looks like this (Fig 2):
Fig 2: System schematic
There are two kinds of refrigerant: R12 and R134. They are not compatible. Essentially there is quite a lot of legislation in this area to do with ozone and the greenhouse effect (very possibly gobbledegook, but we have to live with it). Refrigerant also needs to be scrupulously dry and the system needs to have no air in it. What this all means in my view is that a/c systems should be charged and discharged professionally.
One final wrinkle is the air supply. Ideally when the a/c is not running you want a continuous supply of fresh air from outside. With the a/c on, you want to shut this off and recirculate the air in the cabin so that you are progressively cooling the air. I modified a fresh air flap from a production car. With the a/c on, the flap closes. With the a/c off and the ordinary ventilation fans running, it opens.
System performance
Broadly speaking for the system to perform, the bits need to be big enough to do their job; but they also need to be matched. A huge pump and evaporator will be no good with a puny condenser.
For my money, in a '40, the most critical issue is the size and location of the condenser. My initial plan had been to put a fairly small one in the engine bay. The a/c specialist encouraged me to go for a much bigger one positioned on the front of the coolant radiator. Experience suggests that he was right.
The biggest evaporator I could fit into the available space had a 9" square matrix. Again this performs, but I had to modify the panelling over the footwell so that I could install it with the top surface almost touching the fibreglass of the front cover. If I was doing it again, I would be tempted by something a fraction smaller to reduce the amount of sheet metal work that I needed to do.
The pump has a relatively high output - probably greater than pumps supplied with typical aftermarket kits. Being modern it is reasonably efficient and reasonably compact.
The other issue is how much work you are asking the a/c do; in other words, how much heat seeps into the cockpit, which the air conditioning system must extract. My car has a double skinned rear bulkhead, with sound and heat insulation between the skins and under the carpet that covers the bulkhead. The front bulkhead is also partially double-skinned and carefully sealed. The cockpit was well insulated and well sealed before I started.
How I did mine
The steps I took were as follows
· Read up on the subject.
· Acquired some second hand components from a scrap yard and a spare condenser from a friend.
· Decided an overall component layout
· Went and found a small local a/c specialist, showed him the car and talked over my plans
· Threw everything in the bin and started again.
What happened from there on is what I should have done in the first place:
· Agreed an overall component layout with the a/c specialist, wandering around the car with a tape measure
· The a/c man ordered the right condenser, evaporator, pump and fans, which I took delivery of along with a receiver/dryer. (maybe £500 worth).
· I installed some of the bits (If I'd had the time I would have done them all)
· The car then went to the air con shop where the remaining components were installed and all the plumbing connected. On my instructions, all the plumbing was lashed up temporarily with big cable ties, and all the wiring strung together with chocolate-block connectors. The professionals got it to the point where you could run the engine, turn the a/c on and feel cold air coming out. (another £500 +, although a lot of that was fabricating a pump bracket and installing the pump which I would have done myself given enough time)
· I took the car home, tied up the plumbing properly, finished off and secured the electrics, and finalised ducting etc.
Here is what I ended up with:
Layout
The overall layout in my car is straightforward to describe (see also fig 3).
· Condenser mounted on the front of the coolant radiator
· Receiver/dryer up front too.
· Pump bracket attached to the left cylinder head with an extra idler pulley
· Evaporator over the footwell in a bespoke housing
· Most of the electrics mounted on top of the evaporator housing
Fig 3 - general layout of components
Installing it was a lot of work; because the installation was happening after the build. a lot of things had to be moved, and quite a bit of ingenuity was needed to make it work. The system was plumbed entirely with hoses rather than hard lines. The hoses were routed down the left sill between the fuel tank and the fibreglass skin.
Condenser and fans
The old radiator fans were "suck through" ex-Ford Escort items. At the same time as installing the condenser, new "blow through" fans were installed (Fig 4)
Fig 4 Condenser with blow-through fans mounted on front of radiator
Evaporator and fan
This was one of the most complex parts of the installation; look at the evaporator housings in production cars.
Essentially, you have a squat radiator and a fan which need to be in the same tunnel which is in turn installed into the car. I decided to go for a housing fabricated in aluminium. I made a mock-up using cardboard and masking tape, than took it along to a local firm and asked them to make the real thing in welded aluminium (Fig 5).
Fig 5 - evaporator housing & mock-up Fig 6 - Evaporator installed in housing
I took the evaporator along so that they could check for fit. Getting it all in and screwed down was a bit of a monkey-puzzle. I had to move the master cylinder reservoirs and the washer bottle to make room for it. I removed the heater altogether and blanked off the pipes - I found that I rarely used the heater because my 40 is a summer-only car. Fig 6 shows the evaporator installed.
Pump
This was the second biggest item. Fabricating the pump mounting was difficult, complicated by the need to re-route the cooling hoses from the engine. Figs 7 and 8 show the general arrangement:
Plumbing
As already noted the system was plumbed throughout using hose. The hoses were routed inside the left sill and through the left front wheel arch (making sure there was no chance of the wheel chafing on it (See figures 9 and 10)
Fig 9- Hoses in left wheelarch.
Fig 10 - Hoses routed through box section ducts and clamped to chassis to prevent chafing.
Electrics
Fig 11 - fuses and relays controlling the air conditioning.
The electrics started out relatively simple, but gradually became more complex. I won't bore you with the circuit diagrams.
The system has about five relays and three fuses in it. It works as follows:
· The air conditioning is switched on by a three-position switch (off-low-high)
· Switching it on turns the fan on low or high as appropriate and energises the magnetic clutch in the pump.
· It also sets the radiator/condenser cooling fans running continuously on low (fans in series) and closes the flap on the incoming fresh air supply.
· If the air conditioning pressure switch detects excessive pressure, or the ordinary radiator fan thermostat closes, the radiator/condenser cooling fans go to high (fans in parallel)
· If the air conditioning pressure switch detects dangerously high pressure, the system de-energises the magnetic clutch in the pump, so shutting itself down.
· With the air conditioning off, switching on the ordinary cockpit ventilation fans puts the evaporator blower on high and opens the flap controlling the fresh air supply
Much of the wiring is clustered on top of the evaporator housing (Fig 11)
In conclusion
As with much else on a '40 you can decide how much of the job you want to do yourself, and how much you will pay a professional to do. You can also decide how sophisticated you want to be. The system described here seems to work well, although I have yet to try it out in sweltering hot conditions. There is more that I could do, particularly ducting the evaporator outlet. There are I'm sure other ways of doing the job that would work as well or better. This one works for me.
Useful bits and pieces
The work on my car was done by John Court of Auto Aircon (http://www.auto-aircon.com. 01527 520520) in
Haynes do a useful "Techbook" - Automotive Heating and Air conditioning" ISBN 1 56392 381 5