Boil-out load is the transient heat duty needed to raise an ammonia vessel’s liquid pool from a cold starting temperature up to a target bath temperature over a defined cycle, driven by a warm coil submerged in the pool. It differs from a steady-state coil duty because the bath temperature — and therefore the heat transfer rate — keeps changing throughout the cycle rather than sitting at one fixed condition. Boil-out load governs coil sizing whenever the coil has to complete a warm-up, oil boil-out, or defrost within a required cycle time, not just hold a steady condition indefinitely.
Where boil-out duty shows up
The same transient warm-up calculation applies to a few related jobs that all put a warm fluid through a coil submerged in a cold ammonia pool: driving residual oil out of a vessel before maintenance, running a hot-gas defrost cycle, and warming a vessel up ahead of a planned pump-down. Each case typically starts from its own preset assumptions for coil fluid, starting bath temperature, and target bath temperature.
| Scenario | Typical coil fluid | Typical target bath |
|---|---|---|
| Oil boil-out | Warm high-pressure liquid ammonia | [ELI-REVIEW: roughly 45°F, from a starting bath around 23°F] |
| Hot-gas defrost | Hot discharge gas | [ELI-REVIEW: warmed to a few degrees above 32°F to clear frost/ice] |
| Pump-down warm-up | Warm high-pressure liquid or hot gas | [ELI-REVIEW: a target set by the specific pump-down procedure] |
How the transient calculation works
Because the driving temperature difference between the coil fluid and the bath shrinks as the bath warms, boil-out sizing steps time forward in small increments — [ELI-REVIEW: on the order of a 5-second step] — computing coil duty and ambient heat loss at each step, and integrating the net heat into the bath until the target temperature is reached, an equilibrium (asymptote) is hit before the target, or a [ELI-REVIEW: multi-hour timeout, on the order of 6 hours] is reached without getting there. The shell-side (pool-boiling) heat transfer coefficient used in this step is a correlation [ELI-REVIEW: fit and calibrated against a small set of reference designs over roughly −25°F to +30°F bath temperature], so results at higher bath temperatures — which boil-out cycles routinely reach — extrapolate beyond that calibration range and should be treated as a rough estimate, not a precise prediction.
Run it
The Boil-out tab in ColdCalcs' Coil Suite time-steps the bath warm-up for your coil and vessel geometry and shows cycle time and duty on screen, free.
Open the Coil Suite →FAQ
How is boil-out load different from subcooler duty?
Subcooler duty is a steady-state calculation at one fixed operating condition. Boil-out load is transient: it tracks how the bath temperature, and the resulting heat transfer rate, change minute by minute over the course of a warm-up cycle, and reports how long that cycle takes.
What does it mean if the bath temperature asymptotes before reaching target?
It means the coil's duty at the current bath temperature has dropped to roughly match the vessel's ambient heat loss, so net heating has effectively stopped. That signals the coil, as sized, cannot reach the target bath temperature under the assumed conditions and needs to be reworked.
Does hot-gas defrost warm up faster than a liquid-ammonia boil-out coil?
Real hot-gas defrost typically transfers heat faster than a simple single-phase vapor model predicts, because the gas partly condenses on the tube wall. A conservative single-phase estimate is a reasonable starting point for sizing, but actual cycle times for hot-gas service tend to run faster than that estimate.
Does liquid level in the vessel affect boil-out time?
Yes. Liquid level sets how much of the coil surface is actually submerged and wetted by the pool, and it sets the mass of liquid that has to be warmed, so both the achievable heat transfer rate and the total energy required change with liquid level.