In a refrigerated vehicle the setpoint is made equal to the target temperature of the load. The ATP Agreement sets the highest product temperature at -20 °C for ice cream, -18 °C for deep-frozen food, +7 °C for red meat, +4 °C for poultry and rabbit, and +6 °C for pasteurised milk, butter and fresh dairy products, or the temperature on the label. The target on the panel, however, is different information from the temperature measured inside the load.
Four temperatures get confused
| Reading | What it is | Where it is read |
|---|---|---|
| Setpoint | The target given to the unit | Unit panel |
| Supply air | Air leaving the unit | Air path/probe |
| Return air temperature | Air returning from the body | Unit display, record |
| Product core temperature | Inside the load | Probe at a defined point |
Most installations record return air, while decisions are made on product core temperature. Write down the probe position, the time of reading and the point in the load where it was taken.
Pulling colder for a safety margin. Setting below target because the doors will open is a common habit. Cold air reaches the load nearest the discharge outlet first: fresh product there drops below target and can freeze, while product at the back of the body was already on target. Part of the load pays for the margin.
Setpoint and loading temperature work together
Page 125 of the Thermo King Precedent manual says it plainly: the unit is designed to keep the load at the temperature it was loaded at and cannot pull a hot load down, so product is pre-cooled before loading. Lowering the setpoint for a warm load does not fix it; the fix belongs on the dock, on the pre-cooling side.
On mixed loads, the products are separated
One setpoint cannot hold two different targets at once. Product groups in the same body are separated with a fixed bulkhead, a sealed movable partition or a separate insulated container; volume, access and airflow decide which.
- set the unit to the product's target temperature
- name setpoint, supply air, return air and product core separately
- measure product core before and after loading
- check the product nearest the discharge outlet regularly
- leave air space around the load and keep the evaporator inlet and outlet clear
