HEAT EXCHANGER DEVICE FOR THE PROVISION OF REFRIGERATION IN REFRIGERATED VEHICLES, THE MOTOR VEHICLE ENGINE OF WHICH IS OPERATED BY LNG

Abstract

A heat exchanger device provides refrigeration in refrigerated vehicles operated by liquefied natural gas (LNG) which must first be regasified. The great temperature difference between heat-discharging cooling chamber air and heat-absorbing LNG evaporating at up to −161° C. conducts the heat flow via an introduced intermediate medium circulating in a closed circuit to avert the risk of combustible natural gas leaking. The intermediate medium is non-combustible, environmentally-benign liquid heat exchange media having low viscosity. The liquid heat exchange media operating temperature is kept above −85° C. using an additional thermal resistance in the heat exchanger which evaporates the LNG, so that the heat flow flows with sufficient temperature drop. A thin protective dry gas layer formed using sheathing tubes enclosing a tubular heat exchanger's tubes coaxially serves as this thermal resistance. Possibly escaping natural gas is determined by monitoring pressure in the layer, and the LNG supply interrupted.

Claims

1-5. (canceled)

6: A heat transfer device for the provision of refrigeration in refrigerated vehicles, by means of evaporation of liquefied natural gas (LNG), wherein the re-gasified natural gas (NG) is provided for operating the engine of the refrigerated vehicle, on the one hand, and the available refrigeration energy can be utilized for cooling of the refrigerated goods to be transported using the refrigerated vehicle, on the other hand; wherein a ribbed-pipe heat exchanger (3) arranged in the cooling chamber stands in an active connection with a pipe-bundle heat exchanger (9) as an LNG evaporator, with strict spatial separation, whereby heat can be extracted from the cooling chamber, and heat can be conducted away by means of cooling chamber air (1) conveyed by means of a fan and the ribbed-pipe heat exchanger (3), specifically making use of a liquid intermediate medium (5) conducted in a closed circuit, in forced circulation between the ribbed-pipe heat exchanger (3) and the pipe-bundle heat exchanger (9), which medium, as a synthetic heat medium fluid on the basis of aliphatic hydrocarbons, does not freeze at temperatures as low as −85° C. and remains capable of being pumped, is not harmful to the environment in the event of a leak, and is permissible for occasional unintentional contact with foods; wherein a pipe-bundle heat exchanger (9) produced from cryogenic material is used for evaporation of the LNG that flows in via the LNG inlet (14), which exchanger conducts the LNG to be evaporated in the heat exchanger pipes (18) and conducts the heat-transferring intermediate medium (5) from the inlet (12) to the outlet (13) in the mantle chamber, and is structured as a pipe-bundle heat exchanger (9) having a floating head and two mantle paths, whereby the great temperature changes in terms of time and space that occur during operation exclusively cause controllable mechanical stresses; and wherein coaxially arranged protective pipes (21) sheathe the LNG-carrying heat transfer pipes (18) in such a manner that a hermetically sealed interstice that can be filled with a dry gas (22) is formed, the low layer thickness of which, between the pipes (18 and 21), is designed in such a manner that here, the thermal resistance (R.sub.th) leads to a temperature drop that excludes freezing of the intermediate medium (5) at the surface of the protective pipe (21).

7: The heat transfer device according to claim 6, wherein the pressure of the hermetically sealed dry gas (22) is selected to be clearly lower than the minimum LNG pressure, whereby it is ensured that the pressure increase that occurs in the event of a leak, due to natural gas entering in, triggers the safety pressure switch (24) provided for this purpose and triggers shut-off of the LNG feed by way of this switch.

8: The heat transfer device according to claim 6, wherein the heat can be transferred from the cooling chamber air (1) conveyed by a fan (2) to the ribs of the ribbed-pipe heat exchanger (3) to which intermediate medium (5) is applied on the inside, at which exchanger the heat transition can be implemented on the outside by means of high ribs and by means of a great flow velocity of the cooling chamber air (1), and the heat transition on the inside can be implemented by means of a high flow velocity of the intermediate medium (5), in such advantageous manner that the thermal resistance (R.sub.th=ΔT/{dot over (Q)}), in other words the quotient of the driving temperature difference (ΔT) that decreases at the resistance and the heat energy ({dot over (Q)}) of the heat transition is minimized, and the temperature of the intermediate medium (5), which is thereby maximized, results in reduced viscosity and thereby in a correspondingly low drive energy of the recirculation pump (8).

9: The heat transfer device according to claim 6, wherein the intermediate medium (5) that transports the heat from the ribbed-pipe heat exchanger (3) to the pipe-bundle heat exchanger (9) is conducted in lines (7) that are provided with insulation (10) and are at least partially flexible, which lines can be separated using quick-lock couplings that shut off on both sides.

Description

EXEMPLARY EMBODIMENT

[0020] In the following exemplary embodiment, the invention will be explained in greater detail using a schematic representation, in FIG. 1, of the heat transfer device structured according to the invention, for the provision of refrigeration in refrigerated vehicles, the motor vehicle engine of which is operated using LNG.

[0021] All of the components subject to a low temperature level due to the LNG consist of cryogenic material. All components at low temperatures are protected from undesirable incident heat by means of insulation.

[0022] The cryogenic LNG carried in the refrigerated vehicle in a storage unit is re-gasified in the pipe-bundle heat exchanger (9) provided for this purpose, so as to power the motor vehicle engine as gaseous natural gas (15). The heat required here for evaporation of the LNG is transferred by means of the heat-discharging material stream of a liquid intermediate medium (5), which medium is conducted in forced circulation in a hermetically sealed heat transfer circuit, using flexible hose lines (7) and a re-circulation pump (8) at the inlet and outlet (12, 13) of the intermediate medium (5) into and out of the mantle chamber, and in turn obtains the heat from the cooling air (1) re-circulated in the transport chamber of the refrigerated vehicle with a fan (2), using a ribbed-pipe heat exchanger (3), as refrigeration energy.

[0023] The LNG evaporator is structured as a pipe-bundle heat exchanger (9) having a floating head and two mantle paths, so that the great temperature changes, in terms of time and space, that occur during operation do not cause any uncontrollable mechanical stresses.

[0024] The cryogenic LNG supplied at the LNG inlet (14) enters by way of the heat transfer hood (16) and is distributed to the heat transfer pipes (18) that carry the LNG, by way of a pipe plate (17), flows to the deflection hood (19) in the floating head of the pipe-bundle heat exchanger (9), is deflected, and flows back to the pipe plate (17), once again conducted in heat transfer pipes (18), and from there gets into the upper part of the heat exchanger hood (16) divided by a partition, and finally reaches the internal combustion engine of the refrigerated vehicle, by way of the outlet connector (15), partially or completely re-gasified, directly or by way of an additional heat exchanger operated with waste engine heat.

[0025] All of the heat transfer pipes (18) of the pipe-bundle heat exchanger (9) that carry LNG are surrounded by coaxially running protective pipes (21), which are carried by additional pipe plates provided for this purpose, so that the interstice between the heat transfer pipes (18) that carry LNG and the protective pipes (21), as well as the spaces between the pipe plates at the ends, filled with a dry gas (22) as a protective gas, act as a thermal resistance that is superimposed on the usual transport resistance of the heat transition through a simple pipe wall. A clearly increased driving temperature difference results from the thermal resistance that is thereby increased in targeted manner and clearly counteracts the heat transport from the intermediate medium (5) to the LNG, and this has the result that the surface temperature of the protective pipes (21) wetted by the intermediate medium (5) is clearly greater than that of the LNG, which has a temperature as low as −161° C. With this prerequisite, in other words in the case of a corresponding selection of the layer thickness of the dry protective gas (22), for example nitrogen or air, which thickness is generally very thin, intermediate media (5) are available, which neither freeze nor can no longer be pumped due to overly high viscosity.

[0026] The disadvantage, that because of the increased thermal resistance in the LNG evaporator the heat transition surface has to be increased to ensure the required heat transition, is compensated by the possibility that now exists, of selecting a liquid intermediate medium (5) that is both equally effective and cost-advantageous. In this regard, Therminol D12 is an advantageous solution. This is a synthetic heat medium fluid on the basis of aliphatic hydrocarbons, which is capable of being pumped as a heat medium fluid down to as low as −85° C., which is not harmful to the environment in the event of a leak due to failure, and which is permissible for occasional unintentional contact with foods. The intermediate medium (5) enters into the mantle space of the pipe-bundle heat exchanger (9), divided by a partition plate (20), via the connector at the inlet (12), and flows to the connector at the outlet (13) by way of the resulting two mantle paths, in a flow opposite to the LNG.

[0027] As a further safety-technology measure, it is proposed that the pressure of the dry gas (22) hermetically sealed between the coaxial pipes and in the free spaces between the pipe plates be selected to be clearly lower than the minimum LNG pressure, so that the pressure increase occurring in the event of a leak, due to natural gas entering in, triggers the safety pressure switch (24) provided for this purpose and, via this switch, triggers shut-off of the LNG feed.

[0028] The liquid intermediate medium (5) transfers the refrigeration energy in the transport chamber of the refrigerated vehicle; in other words, it absorbs the heat to be transported to the LNG from the cooling chamber air (1). For this purpose, a ribbed-pipe heat exchanger (3) to which the liquid intermediate medium (5) is applied on the inside and to which the cooling chamber air conveyed by a fan (2) is applied on the outside.

[0029] The heat transition on the outside can be implemented by means of high ribs and by means of the great flow velocity of the cooling chamber air (1), and the heat transition on the inside can be implemented by means of the high flow velocity of the intermediate medium (5), in such advantageous manner that the thermal resistance of the heat transition is minimized and the temperature of the intermediate medium (5), which is thereby maximized, results in reduced viscosity and thereby in a correspondingly low drive energy of the recirculation pump (8) that ensures the forced circulation.

[0030] Furthermore, the environmental friendliness of the intermediate medium (5) ensures that the circuit, conveyed in insulated, partially flexible hose lines (7), can be opened up using quick-lock couplings (6) that shut off on both sides, so as to separate the trailer of a semi-trailer from the truck unit, for example.

REFERENCE SYMBOL LIST

[0031] LNG liquefied natural gas (liquified natural gas) [0032] NG re-gasified natural gas [0033] R.sub.th thermal resistance [0034] {dot over (Q)} heat energy [0035] ΔT temperature difference [0036] PHZ pressure, upper limit value, protection by means of triggering or safety-relevant switching function [0037] 1 cooling chamber air [0038] 2 fan [0039] 3 ribbed-pipe heat exchanger [0040] 4 insulated cooling chamber wall [0041] 5 heat-transferring intermediate medium [0042] 6 quick-lock coupling that shuts off on both sides [0043] 7 flexible line [0044] 8 re-circulation pump [0045] 9 pipe-bundle heat exchanger (having a floating head and two mantle paths) [0046] 10 insulation [0047] 11 insulation [0048] 12 inlet of the intermediate medium into the mantle chamber [0049] 13 outlet of the intermediate medium out of the mantle chamber [0050] 14 LNG inlet [0051] 15 outlet connector for evaporated LNG [0052] 16 heat exchanger hood with partition and LNG connector(s) [0053] 17 pipe plate [0054] 18 LNG-carrying heat transfer pipe [0055] 19 deflection hood (in the floating head of the pipe-bundle heat exchanger) [0056] 20 partition plate (in the mantle chamber of the pipe-bundle heat exchanger) [0057] 21 protective pipe (coaxial to the LNG-carrying pipe) [0058] 22 dry gas (as a hermetically sealed protective gas) [0059] 23 service valve [0060] 24 safety pressure switch (closes the LNG feed in the event of increasing pressure)