1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136 
Body heat must stay constant
Page 63
Page 64
If you've noticed an error in this article please click here to report it so we can fix it.
REFRIGERATION systems fall into two basic categories: mechanical and chemical. The former is the better known as it includes the separate refrigeration units mounted on the front of the standard 40ft box. These separate refrigerators such as Petter and Thermo King are self-contained units operating on a standard evaporation cycle using Freon 12 as the refrigerant. Although relatively complex in the number of components used (eg compressor, condenser, heat exchanger, dehydrator, expansion valve and evaporator coil), the principle of operation is simple.
Freon 12 is a refrigerant gas which is stored as a liquid under pressure in a receiver tank. At the start of the cycle, it is pumped to the expansion valve where the pressure is -reduced, causing the Freon to revert to a gas and remove heat from its surroundings. The evaporator coil is located inside the load space to absorb heat.
The Freon 12 then travels to the compressor and the condenser coil where it is cooled with the aid of an externallymounted fan. Now back in liquid form, the refrigerant is returned to the receiver tank for recycling.
To make the refrigeration unit self-contained, the compressor has its own power source — usually a small diesel engine. Although hydraulic drive can be used, via an engine driven pto, this makes the refrigeration unit dependent on • the prime mover — not a good thing in the event of, say, a motorway breakdown.
The other system which can be included in the mechanical rather than the chemical category is the hold-over plate. In simple terms this is a "cold" plate which is fitted inside the body-work and by gradually warming up keeps the cargo cool. Obviously these plates can only be used on a short-term basis as eventually the plates and the cargo will end up at the same temperature.
The hold-over plate uses a eutectic solution which is frozen before the journey starts. This solution is contained in hollow steel plates which also have tubing running through them carrying a refrigerant gas — the ubiquitous Freon 12.
The hold-over method uses large plates to get the requisite surface area for cooling and _ these are fitted to the bodywork. Although they are detachable if any body repairs are necessary, they normally remain fixed to the body. They are piped up to a mains refrigerating plant overnight ready for use the follow day.
It is difficult to achieve a precise control of the internal body temperature with a hold-over plate but, on the other hand, it is relatively cheap to run. The plates themselves are heavy which — along with their limited range compared with an onboard refrigeration unit — prohibits their use on long-distance -work.
Because it is impossible to "re-cool— the plates once the vehicle is on the move, great care is necessary to keep the temperature increase to a minimum. To prevent a too-rapid warm-up when the doors are open for delivery, double insulated doors are fitted. One point concerning refrigerated transport that cannot be stressed enough is that the equipment — whatever the system or type — is designed to maintain a low temperature in the van.
It is not unknown for an operator to load a (relatively) warm cargo and then turn the refrigeration unit on full. From the outside the cargo might look fully frozen but it definitely won't be on the inside.
Thus frozen cargoes must be loaded at the temperature at which they are to be delivered — the refrigeration system merely prevents a rise in temperature during transit.
Chemical systems In a similar manner to a solid ice cube in a drink turning to a liquid and cooling the drink, so a chemical system depends on a liquid or a solid turning to a gas and absorbing heat. The chemicals used include dry ice (solid carbon dioxide (CO2)), liquid carbon dioxide and liquid nitrogen.
Although it is perfectly feasible to merely include a couple of large lumps of dry ice in the middle of the pre-cooled cargo, most current systems are rather more sophisticated. using an electric fan to circulate air across a bunker loaded with dry ice. A fan has the advantage that t can be switched on and off by a thermostat to improve control.
The advantage of this system is that it is cheap and easy to instal. Its major disadvantage is that it cannot cope with large loads over long distances.
It is very important to remember that carbon dioxide gas will not support life. It is not poisonous — you would die from lack of oxygen rather than by carbon dioxide poisoning. The fact remains however that you would still be dead! Thus a certain degree of caution is necessary when using carbon dioxide — or nitrogen too for that matter. Safety switches are fitted which switch off the fa n(s) when the doors are open.
Another safety recommendation is to allow a few seconds' wait after the doors have bee' opened before entering to alloy the heavier-than-air CO2 to pou out at the bottom.
Liquid nitrogen or carboi dioxide cylinders are installed ii the body. A discharge valvi controls the release of the 'qui( which immediately expands int( a gas.
Insulation
It's pointless spending a lo of money on a refrigeration sys tern without insulating the body The insulating material itself nearly always polyurethane. I can be used in two ways — either cut in slabs which an tailored to suit the vehicle it question, or sprayed on.
There are advantages an disadvantages to both methods The slab polyurethane will givl a higher level of insulation thal the spray, but the cost is highe due to the complication of cul ting the little corner pieces t.1 suit the body. Conversely 011 spray method is cheaper, but i is difficult to get an even coa throughout the body, so th, quality of insulation is inferior.
Any damage to a refrigerate' body should be rectified as soa as possible. This is obviously c vital importance if the body i damaged when on the road an' fully laden.