An insulation calculation works steadily through the whole trip. On a delivery round, though, a large share of the heat arrives not through the walls in that calculation but through a door opened thirty times a day. Turkey's vocational module on refrigerated vehicles calls this the service heat load.
The definition
In the module, the service heat load is the heat that reaches the refrigeration system in addition to everything else because doors are opened during loading and unloading. Its source is the air that enters each time the driver opens and closes the doors of the cargo compartment.
What the air does
The module sets out the mechanism as well: cold air sinks and warm air rises, so the moment the door opens cold air flows out along the floor and warm air takes its place. What happens is more than heat transfer. Two volumes swap their air, the flow continues for as long as the door is open, and a larger door makes it faster.
The variables the module lists
- Door size, the condition of the seals, 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 is running while the doors are open
- Insulation of a roof exposed to direct sun and of a floor exposed to heat from asphalt and engine
Humidity is the invisible line. The module's section on infiltration describes a second load: the water carried in by incoming air condenses, drawing its latent heat of condensation from the space, then gives up sensible heat down to its freezing point, then latent heat of solidification as it freezes, and then sensible heat again down to the temperature of the space. In summer the air is both warmer and wetter, so this line grows. The module also notes how hard this sum is: time-dependent changes and variables that resist measurement make infiltration difficult to calculate.
An icing evaporator and a rising number of defrost cycles are what that water looks like in the field.
When planning chilled product distribution on a multi-stop round, the number of doors matters as much as the length of the trip.
You cannot manage what you do not measure, but you can measure the seconds
Calculating infiltration in watts on a loading bay is not realistic. Measuring how long the door stays open is. Writing down the opening and closing time at every stop for a week produces the real profile of the round. On most rounds the total comes out at twice what people estimate.
| Stops | Door-open time per stop | Total per trip |
|---|---|---|
| 18 | 40 seconds | 12 minutes |
| 18 | 90 seconds | 27 minutes |
| 30 | 90 seconds | 45 minutes |
The figures are an illustration, not your round. The point they make is this: a difference of fifty seconds per stop turns into half an hour of open door per trip.
Two ways to make the door smaller
The first is discipline: prepare the order before opening, take it out in one movement, keep the door shut while searching. The second is architecture: keep the load inside a closed container. Then, when the door opens, the volume exposed to the outside is the vehicle's cargo space rather than the space holding the product. As long as its own lid stays shut, the product is unaffected by how many doors the round contains.
