After calculating the KJ required to heat the steel of a tank, say Q1, plus the KJ to heat up the product, say Q2, and knowing the rate of loss of heat once up to target temperature, say P3 ( and estimating that during the start-up that the rate of loss of heat is 50% of the full rate, due to the temperature gradient during that phase ). We can use (Q1+Q2) / (Latent heat of the steam) = part of the mass of steam required, enabling one to attempt a first pass estimate of the time for the start-up, and then apply that time to convert P3 to KJ also, to then add to Q1 and Q2 for an approx total KJ ....... it would be nice if it were simple and just a function of the steam flow rate. But it is compounded by a heat balance equation, and the unknown loss of heat of the ejected steam / condensate mix plus unknowns such as the product ( bulk liquid ) changing properties during the heat up, eg viscosity, density, thermal conductivity, etc. It seems that one has to resort to empirical data for estimating a start-up duration. Any advice or comments are welcome. There are no specific bulk liquids in question, This is a general issue, in particular for heating coils inside tanks, with steam or even hot water as the heat source. Thank you.