Heating of Flux Bath

A. One of the reasons for heating the flux is to enable better, faster drying of the fluxed steel. The water in the flux will evaporate much faster off warm steel than cool, and this means heated above ambient. This is also dependent on humidity. Having dry steel to immerse in the zinc means less splash and hence less ash (sometimes called top dross or skimmings?). So if your steel is dry before immersion, then this reason for heated flux is discounted.

Another reason is solubility of the ZAC, but at 30 °C that shouldn't be a problem either.

You are lucky with your ambient temps. We have 1 plant where acid temps can go as low as 2 °C in winter - makes for slow pickling. Flux is heated.

A. Dear Francis,
For your application, quadraflux (See Metal Finishing) at 71 °C (160 °F) at about 12 to 14 Deg baumé (Density) will give the best results. If you heat with steam the proper coil material is titanium. If you heat with electricity the heater would be inside a quartz tube and correctly grounded. Both heating methods have a long life if you protect the heaters from product with shields. In use, the steel must remain in the hot flux at least 3 minutes to heat up all the way through so as to properly dry before kettle entry. Quadraflux which is a formulation not a product, dries much better than double salt which has more zinc chloride which is hygroscopic (collects moisture from the air). All fluxes require an effective wetting agent, which only one has passed all the requirements. There are seven flux parameters in flux that must be maintained to get best results.
Properly maintained and used, quadraflux delivers about 30%+ zinc savings, quality that rivals electroplate, and almost never "black" (bare) spots. Zinc "clean-up" after galvanizing is minimal. I liked Jeddah when I was there.
Regards,