Have the process fluid and the heating or cooling medium in front of you, plus the flow and the temperatures in and out. If some of it is still unknown, call anyway; a rough duty is enough to tell whether a stock unit is close.
When a stock size comes close but not exact, you will hear that plainly, along with what a standard or custom unit would change in price and delivery.
Read the nameplate before you call, or photograph it and send the picture by email. The serial number and shell size let us find the matching bundle or O-ring quickly, and a drawing or U-1 settles any question about dimensions.
Bundles and O-rings for the stock BEU heaters are kept on the shelf and shipped quickly, so mention if the line is down.