Heat entering when a door opens: service heat load

Wall heat gain runs continuously, but on a delivery round every door opening at a stop adds a separate heat load. The MEB vocational module on refrigerated vehicles calls this the service heat load (MEB, 2013). ASHRAE notes that infiltration air and the related equipment loads together can exceed half of the total refrigeration load in distribution warehouses.

Variables the module lists

• Door size, seal condition and how long the door stays open
• Outside air temperature and relative humidity
• Wind blowing in a direction that helps warm air flow in
• Whether the refrigeration unit runs while the doors are open
• Insulation of a roof exposed to sun and of a floor exposed to asphalt and engine heat

The module also notes that infiltration is hard to calculate: several variables change with time and are difficult to measure.

Moisture is a separate load

Air entering through the door leaves its moisture to condense inside. According to the MEB module, that water first draws the latent heat of condensation, then sensible heat down to its freezing point, then the latent heat of solidification, and then sensible heat again down to the space temperature. In summer the air is both warmer and wetter, so this load grows.

An icing evaporator and a rising number of defrost cycles are what that water looks like in the field. In the MEB module, defrost is generally not needed at cabin temperatures of 6 °C and above; a reverse-cycle defrost is set there at about 15 minutes per running period of roughly 3 hours, and about 10% of the energy spent on defrost returns to the system as a heat load. Icing or a changed defrost pattern alone, however, does not prove a door cause.

When planning chilled product distribution on a multi-stop round, the number of doors matters as much as the length of the trip.

Measure the time instead of calculating the watts. ASHRAE Handbook Refrigeration, chapter 24, calculates the door air load with the open-time factor Dt, the flow factor Df and the protection effectiveness E (equation 14). Dt follows from the number of passages and the open-close time per passage in seconds; typical times are 15-25 seconds per passage for pull-cord doors and 5-10 seconds for high-speed doors, and as low as 3 seconds.

Df is the ratio of actual air exchange to fully established flow. For cyclically used doors ASHRAE recommends 1.1 below a temperature difference of 11 K and 0.8 above it. E is 0.95 or higher for newly fitted strip or fast-fold doors and can fall to 0.8 on freezer doorways with use and wear; with no protection on a wide-open door, E = 0.

In Gonçalves, Costa and Lopes (2019), most of the empirical models tested overestimated infiltration, and the GO model recommended by ASHRAE was confirmed in their laboratory test. The test rig was two 3 × 3 × 3 m rooms joined by a 0.6 m wide, 1 m high door, so the results hold for those conditions.

Sample data: 18 passages in an 8-hour period at 20 seconds each. Open time is 18 × 20 seconds = 360 seconds, or 0.1 hour, so Dt = 0.1 / 8 = 0.0125.

Door type also changes the result. In a CFD study by Carneiro and colleagues, cited in the same paper, infiltration during opening and closing of a sliding door came out 20% lower than for a hinged door. The figure refers to cold room doors and does not transfer directly to a vehicle body door.

On site, infiltration in watts is not easy to calculate; the time the door stays open can be recorded. The table below shows a recording layout; its figures are sample data, not your route.

A 50-second difference per stop becomes 30 × 50 seconds = 1,500 seconds, or 25 minutes of open door on a 30-stop trip. Compare your own trip records rather than estimates, and do not present a total as a model-specific capacity.