01The process block — five numbers
Medium
Water, oil, air, gas, or a named fluid. If it is a mixture, a solvent, or contains chloride, say so: it changes the sheath material and the watt density, and therefore the price of the element.
Flow or mass
For a flow-through heater: Nm³/h, m³/h, kg/h or L/min — state which. For a tank heater: the volume and how often it is emptied and refilled.
Inlet and outlet temperature
Both, not the rise. Ambient matters too: an outdoor installation in a cold climate has a colder inlet than the same machine indoors.
Pressure
Design pressure and working pressure. Above atmospheric, the heater becomes a pressure envelope and the shell, welds and test regime all follow from that number.
Duty pattern
Continuous, batch, seasonal, or standby. A bank sized for continuous holding and a bank sized for a 40-minute recovery are different machines at different prices.
If you can only send three of the five, send medium, flow and both temperatures. Those three let a supplier estimate; the other two let them commit.
02The electrical block — two numbers
Supply
Voltage, phase and frequency as they exist at the terminal — 400 V 3-phase 50 Hz, 480 V 3-phase 60 Hz, 240 V single-phase. A heater built for 50 Hz on a 60 Hz plant is a control-voltage and fan problem, not a minor detail.
Available capacity
The breaker or transformer capacity you can dedicate to this heater. This is the number that decides whether the bank can be built in the steps your control needs, and whether a soft-start or power controller is realistic.
Control expectation
On/off, steps, or modulating. Say what the process tolerance is in degrees; the supplier will choose the switching method from that, not from a preference.
Cable entry
Where the supply comes in, what size gland, and whether the terminal compartment must be Ex-rated or weatherproof. This is where installation cost hides.
Send the nameplate of the existing supply if this is a replacement. A photograph of the panel and a photograph of the old heater solve more problems than a specification paragraph.
03The mechanical block — one sketch
Envelope
A dimensioned sketch or a photo with a tape measure: available length, diameter, flange face location, and the direction the outlet must point.
Connection
Flange standard and size, thread, tri-clamp, or weld-end. State the mating face on your side, not just what you want the heater to have.
Mounting
Horizontal, vertical, or into a tank through an existing nozzle. Vertical gas heaters drain differently and horizontal ones need the element support thought through.
Access for service
Can the core be pulled without dismantling the pipework? If not, say so now, because it changes the flange and terminal choices.
A sketch is worth the whole electrical block in saved time. Every custom heater is drawn before it is built, and the drawing is where the price is decided.
04The commercial block — what to state out loud
Quantity and spares. Elements are consumables; ordering two spare elements with the unit is almost always cheaper than an unplanned shutdown, and the thread and length must match exactly.
Delivery window and the shipping mode. A 2.4 m, 600 kg heater is not a parcel: state whether you need road, sea or air, and whether the site can receive an oversize pallet or a lift-in crate.
Packing requirements for your country. Timber packaging needs ISPM-15 treatment for international shipment, and the mark must be on the crate — ask for it explicitly if your customs authority checks.
Documentation you will be asked for: certificate of conformity to the drawing, hydrostatic or pressure test record, electrical test record (hipot and insulation resistance), material declarations for wetted parts, and a wiring diagram in the language your maintenance team reads.
Any code or classification your project requires. State it as a requirement with the standard and the zone or division; a supplier who is not told will design to their own default, and unwinding that after manufacture is expensive.
The last line of the list is the honest one: say what you do not know. "Gas is compressed air at 6 bar, temperature unknown, we want 60°C" is a workable inquiry. A guessed number is not.