Chemical Engineer to design a process and create spreadsheet to change all variables parametrically

We need to let a product (dissolved in water) react using heat at a temperature of 200C (and thus under pressure).
We originally wanted to do the following:
1) Enter the inner tube of a tube-in-tube coil counter-flow heat exchanger [A] to raise in temperature from ambient (say, 20C) to as close as possible to the temperature of the next location: A hot oil tank (200C);
2) Next, the stream will pass through a single tube coil [B] to heat up completely to 200C;
3) Next, the stream will enter the outer tube of the [A] coil in counter flow direction to transfer its heat to the incoming stream. The goal is to cool off the water as close as possible to the ambient temperature before it enters the chiller coil.
4) Finally, the stream will pass through a single tube coil [C] (in a 15C glycol tank) to chill to a temperature of (or close to) 15C, after which it will pass a check-valve to relieve the pressure back to ambient.

What we were looking for is a 2-in-1 calculator:
1) for a tube-in-tube coil
2) for a single tube coil
That allows the input of:
- one overall flow rate (L/min);
- the desired ingoing and outgoing temperatures of all coils;
Creating a list of temperatures vs traveled distance (and a graph) which will tell us:
- How long the coils should be to reach the desired temperatures.

I presume the same calculator can be used to calculate coils [B] and [C]. Maybe would only need a change of heat transfer coefficient.

A big problem we recently discovered with this approach is that a tube-in-tube coil will most likely (or certainly?) have its inner tube be in full contact with the outer tube (not possible to co-axially align the tubes).
This poses a big problem with our product since the fluid stream will stop in such tiny spaces and then it crystallise there, causing a risk of plugging the outer tube.

So we’re exploring alternative ideas here, that might a different calculator.

We still wish to know to know the required length of coil [A], because we may choose to go for a series of straight tube-in-tube exchangers (guaranteed co-axial) which we could just couple with braided steel hoses.

We’re also exploring an idea to mill a spiral or serpentine all the way through in aluminum plates (to simulate a long straight piece) and then mill 2 channels in there, close it up with a top plate and use that as an exchanger. Would you be able to make a thermal profile/calculator of that also?

We’re very open to any other suggestions for this last problem that your experience might cover.

So for now a calculator for the originally described coils [A], [B] and [C] would help us in this stage.