Design & operation of local chilled distribution vehicles
Design & operation of local chilled distribution vehicles
Saturday, 11 April 2009
Steve James discusses the factors that influence food temperatures during local chilled distribution.
There will be few real advances in the design of chilled distribution vehicles until there is a firm understanding of the interaction between the refrigeration system, the vehicle’s construction, the air movement within the vehicle, the external environment, the operation of the vehicle and the temperature of the foodstuff. A project completed by FRPERC as part of a MAFF LINK scheme produced a predictive model designed to assist fleet operators in specifying the design of and the equipment for small delivery vehicles.
Refrigerated vehicles are developed and tested in carefully controlled conditions. Due to the large number of interacting variables as many as possible are held constant during the tests. The prediction programme allowed systematically altering one or more variables whilst simulating the operation of a vehicle in a complex, realistic way.
The verified model provided valuable data on the factors influencing food temperature and van performance.
Van Insulation
The heat extracted by the refrigeration system during the journey is shown plotted against the thickness of insulation below. Only a small thickness of insulation greatly reduces the amount of heat that has to be extracted, the amount decreasing with the reciprocal of the thickness of insulation. In all cases, van and food temperatures were maintained at less that 5°C. This was only achieved in the case with no insulation by fitting the vehicle with a refrigeration system with a nominal capacity of 10 kW. The food in the van modified the action of the thermostat and reduced the running time of the refrigeration system and the heat extracted by it. The reduction was 43% when the insulation was 75mm thick.
The heat extracted by the refrigeration plant during the standard journey for vans with different thickness of insulation, with and without food
Infiltration
The heat extracted from a poorly sealed van was 86% more than from a well-sealed van. However, infiltration during the time that the door is closed is a relatively small proportion of the total refrigeration load. In this vehicle, fitted with a nominal 2 kW cooling system, the state of the seals did not cause the temperature of the food to increase to more than 5°C during the journey.
The heat extracted by the refrigeration plant during the standard journey for a well, and a poorly sealed van
Weight of Fittings and Thermal Mass of Lining
The weight of the fittings and the thermal mass of the lining form a sizeable refrigeration load and take a finite amount of time to cool. However, in a sales van this normally takes place late in the evening or in the early hours of the morning when ambient temperatures are low and no other loads are imposed on the van. The load is therefore smaller than the size of the refrigeration plant fitted and the short pull down time from 28°C - 5°C would not warrant keeping the refrigeration system running all night. However, if the vehicle was in continuous use for the transport of foods at different temperatures (chilled and frozen) then pull-down times between changing loads could be a serious disadvantage.
Door Openings
The heat extracted from a closed van is very small. Door openings greatly increase the heat load and, when the van engine drives the refrigeration system, this extra heat must all be removed during the period when the van is moving. Several factors interact when the number of door openings increases. The complete journey takes longer and during the extended journey the ambient temperature and the solar radiation on the van is different from the early part of the journey. If the length of time that the door is left open at each stop is also increased from 5 to 10 min then the temperature of the air around the food in the van increases more during each stop. The refrigeration plant therefore operates at a higher evaporating temperature (and hence it has greater capacity) when reducing the temperature once the doors are closed and the vehicle starts moving again. The time during which the refrigeration plant can run remains the same as the number of drops increases and therefore the rate of heat extraction increases approximately linearly with the number of stops.
The rate of heat extract from the van, averaged over the periods when the vehicle is moving, as a function of the number of stops the van makes
Initial food temperature
The heat extracted by the refrigeration system is 4 times greater if the food is loaded at 7°C than if it is loaded at 0°C. In the case of the predicted the food was spread out over the shelves of the van and so cooled down quickly. If the food had been stacked on the floor with little or no air circulation through the food then the heat extracted would have been less, but the food would have remained warm.
The heat extracted by the refrigeration plant during the standard journey when the food is loaded at different initial temperatures
Length of journey
As the length of the journey gets shorter while the number of drops remains the same the heat entering the van during the stops must be extracted in shorter time intervals between each stop. The rate of heat extraction therefore varies inversely with the length of the journey. It is easier to maintain food temperatures on long journeys than when there are a large number of stops with little time spent travelling between each stop.
The rate of heat extract from the van averaged over the periods when the vehicle is moving as the length of the journey decreases
Solar radiation
A journey was simulated in which a large refrigerated vehicle, designed for carrying frozen food on a long journey The reflectance of the outer surfaces of the trunker had little effect on the heat extracted by the refrigeration system and none on the temperature of the food. However when the vehicle is moving the solar radiation absorbed at the surface is convected away into the ambient air much quicker and significantly reduces the heat load on the refrigeration system compared to that of a stationary vehicle.
The heat extracted from a trunker with 2 different surface reflectance’s and when moving and stationary for the same period
Remember
•In general refrigerated transport is not designed to extract heat from product and the product should be fully chilled before loading.
•There are substantial difficulties in maintaining the temperature of chilled products transported in small refrigerated vehicles that conduct multi-drop deliveries to retail stores and caterers.
•During any one delivery run, the chilled product can be subjected to as many as fifty door openings, where there is heat ingress directly from outside and from personnel entering to select and remove product.
•The design of the refrigeration system has to allow for extensive differences in load distribution, dependent on different delivery rounds, days of the week and the removal of product during a delivery run.