In a passive cold chain no cold is made on the road. Eutectic plates or dry ice are prepared before the trip, the insulated container holds that cold beside the goods, and no unit runs on the vehicle.
How ATP defines the set-up. ATP Annex 1, point 2 defines refrigerated equipment: an insulated body whose cold source is not a mechanical or absorption unit, such as natural ice, eutectic plates or dry ice. With an outside temperature of +30 °C, such equipment must be able to bring the empty body down to its class temperature and hold it there.
| Class | Inside temperature limit (empty body, outside +30 °C) |
|---|---|
| A | +7 °C at most |
| B | -10 °C at most |
| C | -20 °C at most |
| D | 0 °C at most |
Classes B and C require a K coefficient of 0.40 W/m²K or less. Class IR is an insulation category in ATP Annex 1, point 1, with the same 0.40 W/m²K limit. GEBHARDT presents the Isotec TBX in this category, as ATP class IR; the cold source is the plates placed inside it. What makes a set-up passive is the cold source itself.
GEBHARDT Isotec TBX: plates and freezing
GEBHARDT keeps the cold chain for up to 24 hours with eutectic plates. The melting point of a plate is chosen to match the temperature of the goods: -3, -14, -18 or -21 °C. The 390 × 295 × 34 and 615 × 378 × 34 mm plates are standard at -3 and -21 °C; the 480 × 280 × 34 mm plate comes in all four melting points.
GEBHARDT freezes its plates at least 5 °C below their melting point. For a -21 °C plate, the freezer must reach -26 °C or colder. Plates are frozen on racks that leave an air gap between them; a plate that is not fully frozen starts the trip with less cold than the route needs. Freezer targets and times per melting point are given under freezer capacity for a plate fleet.
How much heat enters the container. Here is an example. For a TBX 410 with external dimensions of 580 × 803 × 1,799 mm, the surface area is about 5.91 m². With 30 °C outside, 2 °C inside and the upper k limit of 0.40 W/m²K, heat entry is 0.40 × 5.91 × 28 = 66 W. That is 1.9 MJ (0.53 kWh) over eight hours and 5.7 MJ over 24 hours.
This is an upper estimate: if the k value is below 0.40, heat entry falls in proportion. The mass and fill of the plates set how long the cold lasts, and GEBHARDT's thermal analysis calculates the transport time for a given route. Send us the route and product details and we obtain the analysis from GEBHARDT and pass it on to you.
What a trip involves
A trip runs in four steps:
- Plates are frozen at least 5 °C below their melting point.
- The container is pre-cooled for 12-20 hours: 0 to +2 °C for the fresh zone, -21 to -25 °C for the deep-freeze zone.
- Goods are loaded at their own temperature, and the door is opened only when needed.
- On return the container is cleaned and the plates are frozen again.
A container that is not pre-cooled spends the plate's cold on its own walls first. A passive container does not chill warm goods, because it makes no cold.
Door openings and duration
Each time the door opens, warm air enters; this adds to the heat entry in the calculation above and shortens the time the cold lasts. The 24-hour figure should therefore be read together with the number of door openings. If a route runs beyond 24 hours, its duration is checked separately with the thermal analysis.
