Anyone who has struggled with getting a campfire or a traditional fireplace going knows that managing combustion is harder than it looks.

Watlow, a designer and manufacturer of complete thermal systems, signed a definitive agreement to sell to Tinicum L.P., a private-equity firm. The deal is expected to close as quickly as reasonably possible pending receipt of customary regulatory approvals. Watlow’s products include a range of industrial heaters, thermocouples, RTDs, power controllers and data loggers. The company’s headquarters will remain in St. Louis, Mo.