TURB PENETRATION SPD OF B737-800 CONFLICTS WITH DEP CTLR SPACING REQUIREMENTS.

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ON DEP FROM DFW PASSING 6000 FT A B757 AHEAD OF US ON A DIFFERENT FREQ CALLED BACK TO WARN US OF 'STRONG MODERATE TURB' ON CLBOUT. WHEN AN ACFT FLIPS BACK TO A PREVIOUS FREQ TO WARN YOU; YOU KNOW YOU HAD BETTER TAKE IT SERIOUSLY. SO I DIRECTED THE FLT ATTENDANTS TO STAY SEATED AND DIRECTED THE COPLT TO ACCELERATE TO 280 KIAS TURB PEN SPD. WE ADVISED DEP WE WOULD BE AT 280 KIAS DUE TO TURB; AND AT 11000 FT WE ENCOUNTERED STRONG MODERATE TURB. DEP TOLD US TO SLOW TO 260 KIAS AND WE ADVISED THEM WE WERE UNABLE DUE TO TURB. HE SENT US TO A DIFFERENT FREQ. THAT CTLR THEN TOLD US TO SLOW TO 250 KIAS. WE TOLD HIM WE WERE UNABLE DUE TO TURB. HE CAME BACK IN A 'STRONG' VOICE AND TOLD US RATHER DIRECTLY TO SLOW TO 250 KIAS. I TOLD HIM AGAIN STRONGLY I WAS UNABLE TO SLOW DUE TO TURB. SENSING HE COULDN'T FIND A SOLUTION TO HIS SPACING PROB; I SUGGESTED HE PUT US ON A VECTOR. HE DID. AS THIS CTLR SEEMED UNAWARE OF ACFT PERFORMANCE ISSUES IN TURB; I ATTEMPTED TO BRIEFLY EDUCATE HIM BEFORE I LEFT THE FREQ. HE DIDN'T SEEM VERY RECEPTIVE AND STATED I SHOULD DECLARE AN EMER IF I REFUSED TO FOLLOW HIS INSTRUCTIONS. IT DOESN'T SEEM VERY PRACTICAL TO DECLARE AN EMER EVERY TIME I AM UNABLE TO COMPLY WITH AN INSTRUCTION. PERHAPS A BETTER IDEA IS TO PROVIDE CTLRS WHO WORK SEPARATION CRITICAL AREAS LIKE CLASS B APCH/DEP AREAS WITH TRAINING IN ACFT PERFORMANCE IN TURB. ANOTHER IDEA IS TO INCREASE SPACING WHEN ACFT RPT MODERATE TURB. THIS WILL ALLOW CTLRS MORE 'SAFETY FACTOR.' I UNDERSTAND THE CAPACITY NEEDS OF ARPTS SUCH AS DFW; BUT THESE DON'T OVERRIDE SAFETY. TRYING TO MEET CAPACITY NEEDS BY SPACING ACFT TOO CLOSE TOGETHER IN TURB IS SURE TO LEAD TO MORE CONFLICTS SUCH AS THIS ONE.