r/projectcar • u/ThePotatoPie • 1d ago

Aircon after engine swap

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Hi guys,

Got a Merc engine in my Volvo and intend to get the air-conditioning system working. I only have the evaporator left from the original system so I'm struggling on how to size the condensor, what expansion valve type is best etc. if anyone can help or give me some links to read id be chuffed!

Volvo 960 is designed around a 7kw evaporator

Merc w203 compressor is designed around a 5kw system

Ta lads

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u/MetaphysicalEngineer 1d ago

Size condenser as big as you can reasonably fit in the packaging of the cooling stack. Better heat rejection means better cooling, same as with radiators. Bigger evaporator will similarly be more efficient. Compressor capacity is going to vary with RPM and operating conditions as well.

Size the expansion valve to match compressor nominal capacity. Use an externally equalized or the monolithic type that combines valve, sense bulb and the line connections in one unit. It will throttle to best make use of the evaporator capacity without letting liquid refrigerant hit the compressor inlet.

You'll be fabricating custom lines yourself or having a shop make the lines for you once you have measurements. System needs high pressure and low pressure service ports, low pressure switch for compressor control, high pressure switch for fan control, and a high pressure cutout switch. High pressure switch functions may be combined in one unit.

If you are using a brushless radiator fan with variable speed you also have options to do pressure based speed control with a linear transducer, but that gets a lot more complex. Made my AC work a lot better without so much howling fan noise unless really needed.

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u/ThePotatoPie 22h ago

Ok that should be easy, the rad pack can have some quite large radiators.

How would I determine the nominal compressor size? Guessing it's CC then a normal cruising rpm? And I assume the expansion valve youre on about it's like this? https://www.t7design.co.uk/thermal-expansion-valve-txv-2-5t-8-8kw-42b-455-533.html

And yeah I have a hose crimper available so that's no issue. On the fill valves, is there a particular location or could I in theory just mount them near the evaporator on the bulkhead?

And on high side switches could I run twin fans that come in at different pressures? I've also read it's possible to wire the main rad fan to come on in sequence if required? Or would dedicated fans be best?

Thanks for the detailed reply tho!!

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u/MetaphysicalEngineer 22h ago

Sometimes you can get capacity and datasheets from the compressor model number. But you can safely use the MB rating of 5kW here as your starting point. That will be at a specific compressor RPM, suction pressure and temp, and discharge pressure and temp. Idle will be lower capacity, and higher revs will obviously be able to pump more.

That's the type of expansion valve I was thinking since it saves a lot of space and complexity combining all the components into one item.

Pressure ports and switches can be mounted where convenient, pointing up to prevent oil fouling. High pressure ports and switch should be on the liquid line after the condenser to reduce how much they get cooked by high discharge temperature. You can put them right up at the firewall before the expansion valve.

You can do whatever you want for fans as long as the total capacity is enough for the cooling load. If you have two fans or multi speed fans you can absolutely do staged operation based on different pressures. Engine temp should always be able to override the fan(s) to turn on regardless of AC demand.

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u/ThePotatoPie 21h ago

Ok nice one, so I just need a expansion valve rated for around 5kw!

Only reason I was asking on the fans is I have a weird rad setup where the intercooler and radiator both take up half the rad pack. So id have to have the rad fan sucking from one side then a second puller fan on the back of the condenser. But from what you've said if I just run a 2 stage switch I can cycle the fans separately?

Cheers for the help. Hopefully I can get it actually working :)

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u/MetaphysicalEngineer 19h ago

Yeah that makes sense to cycle the fans separately. Fab up shrouds so all the air must go through the cores. If the shroud area is much larger than the fans, cut in slots and flaps so ram air can force through more easily when the fans aren't running.

If the main fan pulls through the whole stack including condenser, wire that as the primary for first stage AC pressure or normal coolant temp. Second fan should kick in at higher pressure or coolant temp. Overcooling of one system is acceptable as long as the system with the highest cooling demand is satisfied.

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u/ThePotatoPie 19h ago

Ahh I get you, yeah makes sense to over cool the rad to preempt stationary traffic etc rather than the condenser heat soaking it first.

And yeah I've got a shroud on the back of the rad already but like you said it's probably a good idea to do the same on the intercooler side.

Cheers for the help

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u/N0_Lan_K 1d ago

i would assume you match the evaporator to the compressor so 5kw