zz les . ▁The ▁temperature ▁in ▁the ▁jack et ▁is ▁reg ulated ▁to ▁control ▁he ating ▁or ▁cool ing . ▁ ▁The ▁single ▁jack et ▁is ▁probably ▁the ▁oldest ▁design ▁of ▁external ▁cool ing ▁jack et . ▁Despite ▁being ▁a ▁tried ▁and ▁tested ▁solution , ▁it ▁has ▁some ▁limitations . ▁On ▁large ▁vessels , ▁it ▁can ▁take ▁many ▁minutes ▁to ▁adjust ▁the ▁temperature ▁of ▁the ▁fluid ▁in ▁the ▁cool ing ▁jack et . ▁This ▁results ▁in ▁sl ugg ish ▁temperature ▁control . ▁The ▁distribution ▁of ▁heat ▁transfer ▁fluid ▁is ▁also ▁far ▁from ▁ideal ▁and ▁the ▁he ating ▁or ▁cool ing ▁tends ▁to ▁vary ▁between ▁the ▁side ▁walls ▁and ▁bottom ▁d ish . ▁Another ▁issue ▁to ▁consider ▁is ▁the ▁in let ▁temperature ▁of ▁the ▁heat ▁transfer ▁fluid ▁which ▁can ▁oscill ate ▁( in ▁response ▁to ▁the ▁temperature ▁control ▁val ve ) ▁over ▁a ▁wide ▁temperature ▁range ▁to ▁cause ▁hot ▁or ▁cold ▁sp ots ▁at ▁the ▁jack et ▁in let ▁points . ▁ ▁Half ▁co il ▁jack et ▁ ▁The ▁half ▁co il ▁jack et ▁is ▁made ▁by ▁w eld ing ▁a ▁half ▁pipe ▁around ▁the ▁outside ▁of ▁the ▁vessel ▁to ▁create ▁a ▁semi ▁circular ▁flow ▁channel . ▁The ▁heat ▁transfer ▁fluid ▁passes ▁through ▁the ▁channel ▁in ▁a ▁plug ▁flow ▁fashion . ▁A ▁large ▁re actor ▁may ▁use ▁several ▁co ils ▁to ▁deliver ▁the ▁heat ▁transfer ▁fluid . ▁Like ▁the ▁single ▁jack et , ▁the ▁temperature ▁in ▁the ▁jack et ▁is ▁reg ulated ▁to ▁control ▁he ating ▁or ▁cool ing . ▁ ▁The ▁plug ▁flow