Evaporator Design TD and Humidity Band
The design split between box temperature and suction, and the humidity it holds in the box.
Example
You enter
- Box (room) temperature (°F) 35
- Saturated suction temperature (°F) 25
You get
- Design TD (DTD) 10 F
Checked against published worked examples from: Heatcraft Refrigeration Products.
Details, formula, and sources
ScopeDTD = box temp - saturated suction, which sets the box humidity: 7-9 F ~90% RH (produce), 10-12 F ~80-85% (packaged meats), 12-16 F ~65-80%, 17-22 F ~50-65%. A 35 F box on a 25 F SST -> 10 F TD, ~80-85% RH (Heatcraft's beef-cooler example); a drying 17 F suction -> 18 F TD, ~50-65% RH. A smaller TD needs more coil. The coil rating at the design TD governs.
Formula or tableDTD = box_temp - saturated_suction_temp; humidity band by DTD (Heatcraft classes: 7-9 F ~90% RH, 10-12 F ~80-85%, 12-16 F ~65-80%, 17-22 F ~50-65%).
Source and editionThe evaporator design temperature difference (DTD) and the resulting box-humidity bands, from refrigeration coil-selection practice, by name.
Free accessThe DTD-to-humidity relationship is standard published refrigeration coil-selection guidance.
What governsEstimate. AHJ and licensed professional govern.
API field namesbox_temp_f, sst_f, dtd
- Design TD DTD = box temperature - saturated suction temperaturecoil-selection practice
- Humidity bands 7-9 F ~90% RH, 10-12 ~80-85%, 12-16 ~65-80%, 17-22 ~50-65%Heatcraft Engineering Manual H-ENGM0408 (2008), p. 21
- Coil tradeoff a smaller DTD holds more humidity but needs a larger coilscope of this tile