Portable Camping Water Heater
Hit a $200 budget with 53 cents to spare, and specified a pump that could not prime
What I did on this
I did the thermal calculations and the bill of materials, and shared the alternatives analysis. Another member of the team built the CAD model.
total build cost, against a $200 ceiling
53 cents of margin
packed mass, against a 6 kg ceiling
325 g of margin
heat needed to raise the water 40 °F
The problem
Camping somewhere primitive means no hot water, and a design brief with hard numbers attached:
| Constraint | Target |
|---|---|
| Water temperature | 90–105 °F, and never above 110 °F |
| Total cost | under $200, including parts, labour and shipping |
| Packed size and mass | fits a backpack — under about 6 kg and 30 L |
| Capacity | at least two full showers without restocking |
What I did
The team worked through a wide set of alternatives before committing to one — solar bags, chemical heaters, phase-change material, an electric coil, body heat, heated rocks, a pedal-driven generator, and a propane-fired coil. Propane won on energy density: nothing else stores enough heat per kilogram to run two showers out of a backpack.
Sizing it came down to a few calculations.
How much heat. Raising the water about 40 °F at the flow rate the pump delivers needs roughly
20,000 BTU, from q = flow rate × 500 × ΔT.
Whether the propane lasts. The burner runs at 50,000 BTU/hr and a pound of propane holds about 21,500 BTU. Two five-minute showers use around 39% of one cylinder, which leaves a comfortable margin rather than a marginal one.
How much coil. Nine turns of ½ in OD copper at 4 in diameter gives 1.21 ft² of surface and a 4.5 in stack. Nine was the most turns that stayed under 10 ft of tube, which was the budget limit rather than the thermal one.
Whether it is safe to hold. One inch of ceramic fibre insulation rated to 2400 °F, with
k ≈ 0.44 BTU·in/hr·ft²·°F over 1.33 ft², keeps the outer wall cool enough to sit next to a
flame.
How it turned out
It comes in at $199.47 against a $200 ceiling, and 5.675 kg against 6 kg. Both constraints are met, and both by less than half a percent — which is worth saying plainly rather than presenting as precision. There is no margin in either number. A single price change or a heavier substitution breaks it.
But it does not work as specified, and the reasons are worth more than the budget was.
The water pump we selected needs to be submerged to prime, which nothing in our layout does — our instructor pointed that out. It is a specification error rather than an arithmetic one: we chose the pump on flow rate and mass and never checked its installation requirement. It was on the bill of materials, costed and weighed, and it would not have moved any water.
The heat exchanger calculation has a second, quieter problem. Rearranged as Q = U·A·LMTD, the
overall heat transfer coefficient for gas-to-water sits somewhere between 20 and 300, and the
required log mean temperature difference swings from 826 °F at U = 20 to 55 °F at U = 300.
That is more than an order of magnitude of uncertainty in the term the whole design rests on. The
propane budget and the coil geometry are sound; the claim that this specific coil transfers enough
heat is bounded, not established.
What I'd do next
Two things, and the cheap one first.
Re-select the pump against its full datasheet, not two numbers from it. The failure here was a process failure — we filtered the options on the specs we had thought to compare, and installation requirements were not among them. The fix is a checklist that includes how a part has to be mounted, not a better pump.
Measure U rather than bracket it. A single bench run — water in at a known temperature, out at a known temperature, at a known flow rate — collapses that whole range to one number and turns the sizing from an estimate into a result. Everything else in this design is arithmetic; that is the one term nobody actually knows.