Project Niflheim was a domain meant to create a cold environment antithetical to beasts who couldn't handle the cold whatsoever. Fortunately, even before he got to the project, Project Muspelheim had given him several ideas.
"Convection," Rui remarked. "It's more useful for getting rid of heat than heating."
While Rui was confident that he could use it to heat the Muspelhein domain, it was even more naturally useful in the Niflheim domain. He could use convection to filter out heated air allowing only cool air to remain within his domain, significantly reducing the temperature.
In fact, it was precisely this principle that was used in modern cooling units to prevent equipment, electronics, and machinery from burning or melting. Thus, its cooling power could not be understated.
Yet, that was just one of many cooling elements that Rui had in mind.
"If high pressure yields high temperatures, then low pressures will yield low temperatures."