A B737-300 IN CLB AT 8000 FT RETURNED TO THE FIELD DUE TO FAILURE OF THE L AIR-CONDITIONING PACK AND WITH THE MISTAKEN IMPRESSION THE R AIR-CONDITIONING PACK WAS ALSO DEFERRED INOP.

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ACFT HAD 2 MELS. ONE RELATED TO A PURGE VALVE ON THE R AIR-CONDITIONING PACK. MAINT HAD PLACED AN 'INOP' PLACARD NEXT TO THE R PACK SWITCH IN THE COCKPIT. AFTER REVIEWING THE MEL; WE WERE LED TO BELIEVE THAT THE R PACK WAS INOP AND VERIFIED THAT THE FLT WAS FILED AT 25000 FT FOR SINGLE PACK OP. AFTER TKOF; THE REMAINING OPERATIVE PACK FAILED. THIS RESULTED IN A LOSS OF CABIN PRESSURIZATION AND NECESSITATED A RETURN TO THE DEP ARPT. AFTER RETURNING TO THE GATE; MAINT INFORMED ME THAT THE MEL ON THE R PACK ONLY PERTAINED TO A MAINT FUNCTION AND THAT THE PACK SHOULD HAVE BEEN OPERATED NORMALLY. HAD THIS BEEN THE CASE; THE FLT COULD HAVE CONTINUED TO ITS DEST WITH THE SUBSEQUENT INFLT FAILURE. THIS OCCURRENCE WAS A RESULT OF A VAGUE MEL THAT; UNLIKE THE MAJORITY; HAD NO OPERATIONAL EFFECT ON THE SYS. OBVIOUSLY; IT WOULD BE HELPFUL IF MAINT COULD EITHER REFRAIN FROM PLACING AN 'INOP' PLACARD ON A NORMALLY FUNCTIONING SYS; OR PROVIDE AN ANNOTATION TO THE MEL STATING THAT THE INOP COMPONENT DOES NOT AFFECT THE FLC.