How did this happen? (WARNING- graphic photos! ONLY for the automotively mature!)

[ROLLGUY]

How did this happen? What caused it?
The engine was idling and a major explosion happened, shattering and sending the lower grill (W211) yards away, and breaking the splash shield below the radiator in half. The receiver/drier opened up like a tin can (presumably at the seam).

[speednjay]

There’s a lot of psi in the ac system. It’s possible.

[choprboy]

The liquid receiver tank ripped along the seam and burst wide open... In operation that is filled with liquid refrigerant at pressures of 200+ PSI (on a really hot day it could 300+). Possibly an overfilled system, a defective high pressure cutout switch, and/or defective/damaged receiver weld.

A tiny defect can lead to a burst which almost instantaneously rips down the seam. The refrigerant expands 100 fold, flashing from a liquid at high pressure to a gas at atmospheric pressure, and goes off like a bomb. Like a plugged air compressor tank without safety relief... when it gets the bursting point it releases explosively.

[engatwork]

Wow, good pics. You exceeded it's ability to hold pressure. Something is amiss though because the relief valve on the compressor should have dropped pressure before exceeding max allowable on the dryer. I say this not knowing dryer source/quality. Make sure you insure nothing is plugged during repair.

Hasn't it been pretty hot in your area? Make sure your electric fan is operating as it is supposed to too.

[ROLLGUY]

I replaced both the radiator and condenser/drier unit. I just received the pressure switch, and will be installing it soon. I don't know if there is a relief valve in the system of a 211 chassis, but maybe just a pressure switch. Since the compressor pulley is turning whenever the engine is running (no electromagnetic clutch), it may have damaged the compressor. I am not exactly sure how these systems work. And yes, it was over 100 degrees the day the drier blew up. There was a crowd around the car after the explosion, all wondering if everyone was okay, and what exactly happened.

[engatwork]

Relief valve is typically attached to compressor.

I suspect it made a heckuva racket.

[123boy]

Possible contributors to this catastrophic failure: Malfunctioning pressure relief valve (highly unusual if valve not present). Malfunctioning "high pressure" switch/cut out.
Obstructed or plugged filter/dryer. Defective part(s), as previously mentioned. High ambient temps, in conjunction with other defects, could be a likely culprit.
And, not to be ignored and absolutely critical, the proper charge (by weight) for the system being discussed/diagnosed. The accurate freon charge is paramount to the proper operation and longevity of the various components. I'm not suggesting the OP is doing anything wrong...stuff happens. However, I've seen various posts of people who just "charged" their system and they include a picture of a thermometer, stuck in the dash vent, reading temps in the mid to low 40's. This is not good. That's too low! Keep in mind that the vent temp is normally about 20 degrees HIGHER than the evaporator temperature. If your vent temps are in the 40's, that means your evaporator core is in the 20's, which is below freezing and "no bueno". The evaporator could freeze, turning into a block of ice, and cause other issues.
Is it possible to "shotgun" a freon charge? Absolutely. But not without a proper set of gauges (to read BOTH high and low sides), prior experience and the knowledge to tell you what you're seeing, what you need to see and if everything is in the "ballpark".
If nothing else is gained from this "rant", please be careful and USE CAUTION around freon systems. ALWAYS PROTECT YOUR EYES AND WEAR GLOVES. Freon under pressure can cause nasty burns. And, as in this case, produce an exploding bomb capable of launching shrapnel and projectiles who knows where.
Be safe, friends.

Edit: Pressure relief valves can also be internal to the compressor. The internal valve allows the high side to vent to the low pressure side, thereby equalizing the pressure in the system and the threat of catastrophic failure. Not sure how common these internal relief valves are in automotive ac.