GB2131094A

GB2131094A - Engine oil heat recovery system

Info

Publication number: GB2131094A
Application number: GB08330689A
Authority: GB
Inventors: George Albert Coffinberry
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1982-11-29
Filing date: 1983-11-17
Publication date: 1984-06-13
Also published as: FR2536842A1; IT8323794A0; DE3342591A1; JPS59113240A; GB8330689D0

Abstract

Heat from hot oil in an engine 6 is transferred by a heat exchanger 12 to a coolant circuit 18 which transfers the heat through further heat exchangers 33, 36 to fuel for preheating prior to combustion in the engine and to fuel in fuel tanks 51. The amounts of heat transferred out of the coolant circuit 18 are controllable to prevent overheating of the fuel. If the fuel has insufficient capacity to absorb heat from the coolant, an airstream can be connected to a heat exchanger 27 to absorb excess heat. The oil may be from the transmission gearbox 3 of a turboprop engine 6, the coolant being a water/antifreeze mixture. <IMAGE>

Description

SPECIFICATION Heat recovery system The invention relates to heat recovery systems and, more particularly, to such systems which recover heat from a lubricating oil in an engine and use the heat to preheat fuel to be burned by the engine.

BACKGROUND OF THE INVENTION When a relatively high speed engine, such as a gas turbine engine, is used to drive a relatively low speed load, such as an airplane propeller in a turboprop system, the high speed of the engine is commonly reduced by means of a gear transmission. The gear transmission contains a lubricating oil which becomes heated due to factors such as friction and agitation inherent in the gearbox machinery. The heat produced is commonly conducted to the surroundings as waste. Since such transmissions are approximately 98 percent efficient, and since the largest inefficiency is in heat loss, the waste heat produced by the transmission amounts to about 2 percent of the engine output power.

Further, during high speed operation, greater heat is produced and active steps must be taken to eliminate this heat from the transmission. One of the most frequent methods used is to direct an airstream bled from the engine or ducted from the propeller onto a heat exchanger through which the transmission oil is circulated. The latter ducting is shown in Fig. 1. An airstream 101 is ducted from the propeller 104 through a housing 107 and onto an oil cooler 109 through which the oil from the transmission 111 is circulated. However, this method of heat elimination compounds the inefficiency of wasting the heat produced by the transmission 111 because in disposing of the transmission heat, the method diverts airstream 101 which would otherwise be used for providing thrust.Some estimates indicate that the diverted propeller airstream 101 accounts for about 2 percent of the total engine thrust. Therefore, in turboprop engines the combined inefficiencies of the transmission together with the diverted air used to cool the transmission oil amount to about 4 percent of total thrust. This is a significant quantity.

One method of reducing the waste and recovering some of this waste heat by returning the waste heat to the engine cycle is to apply the heat to an oil-fuel heat exchanger to preheat the engine fuel. However, several problems arise in using fuel-oil heat exchangers. One, the fuel is generally under high pressure (about 1000 psi) and there is always a small, but increased, risk of leakage when handling fuel under high pressure as compared with handling low pressure fuel.

Leakage between fuel and oil is to be avoided.

Two, if the fuel becomes too hot, it deposits impurities upon (it "cokes") the fuel nozzles through which it is sprayed. Three, at low engine speed the flow of fuel may be insufficient to absorb enough heat from the cooling oil, thereby allowing the oil to operate at a higher temperature, thus reducing the useful life of the transmission.

An additional consideration concerning fuel heating is the heating of fuel, not as it is delivered to the engine as described above, but when it is contained in the aircraft fuel tanks, in order to prevent freezing. While the fuel itself does not ordinarily freeze at the temperatures to which it is normally subjected, water droplets present in the fuel can freeze and thereby obstruct the fuel handling system. Further, it is envisioned that different fuels with different freezing characteristics will come into use in the future and thus fuel tank heating may require more serious attention.

Therefore, it is desirable to consider the application of the waste heat mentioned above to the purpose of fuel tank heating.

However, problems arise in transporting heat from the engine (if the engine is used as a heat source) to the fuel tanks, at least for the reason that the engine and the fuel tanks are spatially separated. Routing hot transmission oil to the fuel tanks entails the use of extra oil lines and associated equipment and this approach presents an increased risk of transmission oil loss simply for the reason that more fluid circuitry components are involved.

Conversely, routing the fuel from the tanks to the engine is undesirable at last for the reason that a single fuel line running to the engine, namely the fuel supply line to the combustors, is preferable for purposes of safety in fire control. The extra fuel lines needed for fuel heating are undesirable.

OBJECTS OF THE INVENTION It is an object of the present invention to provide a system for recovering waste heat in a turboprop engine system.

It is a further object of the present invention to provide a new and improved lubricating oil cooler.

Accordingly, the invention provides a coolant circuit, a heat exchanger for thermally connecting the coolant circuit with engine lubricating oil, and a second heat exchanger for thermally connecting the coolant circuit with engine fuel.

In the drawings: Figure 1 depicts a portion of a prior art turboprop engine.

Figure 2 depicts one form of the present invention.

Fig. 2 is a schematic representation of one form of the present invention. Transmission oil is routed from a propeller transmission 3, (which is driven by a gas turbine engine 6, and which in turn drives a propeller 7), through a line 9 to a first heat exchanger 1 2 and back to the transmission 3 through a line 1 5. A coolant circuit comprising a line 1 8 carries a coolant such as water and brings it into thermal contact with the transmission oil in the first heat exchanger 12. The coolant absorbs heat from the transmission oil and then travels from the first heat exchanger 1 2 to a second heat exchanger 21 where it can come into contact with a cooling airstream 24. The cooling airstream 24 is tapped by positioning a duct 27 downstream of the propeller 7.The duct 27 is preferably rems-va- ble from the airstream 24 or otherwise able to be shielded from the airstream 24 to reduce the diversion of the airstream 24 from providing thrust when cooling of the second heat exchanger is not needed. A particular means of shielding the duct 27 is considered to be known in the art and is not shown in Fig. 2.

The amount of the airstream 24 which is tapped is controllable by a throttling valve 30 which is preferably closed to prevent diversion of thrust-producing air mentioned in the Background of the Invention. An optional valve 21A can be provided for additional control of the heat transfer through the second heat exchanger 21.

Following the coolant's excursion through the second heat exchanger 21, the coolant sequentially travels along the coolant circuit through a third heat exchanger 33 to the extent allowed by a valve means 34, and then through a fourth heat exchanger 36 to the extent allowed by a valve means 39. A pump 42 circulates the coolant in the coolant circuit 18.

An engine fuel pump 48 raises engine fuel withdrawn from a fuel tank 51 to a high pressure, such as 1000 Ibs psi, and this high pressure fuel passes through the third heat exchanger 33 for absorption of heat from the coolant circuit 18. A pump 55 withdraws fuel from the aircraft fuel tank 51 and sends it through the fourth heat exchanger 36 for absorption of heat from the coolant circuit 1 8 and the fuel is returned to the fuel tank through line 58.

Accordingly, the coolant absorbs heat from the transmission oil in the first heat exchanger 1 2 and then travels to the second heat exchanger 21. The air throttle 30 will normally be closed in order to avoid thrust loss as explained above. However, if insufficient fuel is passing through the third and fourth heat exchangers to absorb enough heat from the coolant circuit 1 8 (an unusual situation), then the second heat exchanger 21 is utilized to absorb heat. In this case, the air throttle 30 is open to the airstream 24, diverting part of the airstream 24 through the second heat exchanger 21 for absorption of heat contained in the coolant by the airstream.As mentioned, the airstream can be tapped from an engine compressor bleed source, such as a static bleed, as well as being ducted away from the airstream 24 of the propeller 7 as shown.

After leaving the second heat exchanger 21, the coolant delivers heat to the fuel traveling through the third heat exchanger 33 en route to spray nozzles 63 in the engine 6.

The third heat exchanger 33 thus preheats the fuel for combustion. In order to control the amount of heat delivered to the fuel to, for example, inhibit coking of the fuel nozzles 63, valve 34 is provided. Valve 34 is preferably automatically controlled as a function of fuel temperature, but normal control by the pilot is possible. Valve 34 controls the amount of coolant delivered to the third heat exchanger 33. Valve 39 similarly controls the amount of coolant delivered to the fourth heat exchanger 36 which heats fuel which is delivered to the fuel tank 51 to prevent freezing.

A first temperature transducer 75 senses the temperature of the fuel traveling to the spray nozzles 63 and provide a signal on lead 77 which is indicative thereof. If this temperature exceeds a predetermined threshold, which is preferably 300OF, then a first valve control 79 associated with the valve means 34 operates to modulate the valve means 34 to adjust the coolant flow through the third heat exchanger 33 so that the temperature of the fuel delivered to the spray nozzles does not exceed 300'F. Thus, coking of the spray nozzles 63 is inhibited.

A second temperature transducer 81 senses the temperature of the fuel present in the fuel tank 51 and provides a signal on lead 83 which is indicative thereof. If this temperature exceeds a predetermined threshold, which is preferably 140OF, then a second valve control 85 operates to modulate the valve means 39 to adjust the coolant flow through the fourth heat exchanger 36 so that the temperature of the fuel in the tank 51 does not exceed 140OF. Thus, evaporation of the fuel in the tank 51 due to excess temperature is reduced.

A third temperature transducer 88 senses the inability of the third and fourth heat exchangers 33 and 36 to absorb heat from the coolant circuit by sensing the temperature of the coolant in the circuit 1 8. When this temperature exceeds a predetermined threshold, such as 3504F (an event that is seen as unusual), the heat sink capacity of the fuel, both in the fuel tank 51 and traveling to the spray nozzles 63 is deemed to be exceeded and a control 93 operates in response to open a valve to the airstream 24 to provide cooling air to the second heat exchanger 21. At this time the thrust penalty incurred in diverting the airstream 24 is tolerated. At other times, the penalty is greatly reduced because the diverting is reduced. The duct 27 is preferably aerodynamically low in drag and preferably removable from the airstream 24 when the second heat exchanger 21 is not in use.

A pressure accumulator 71 is provided in the coolant circuit and it preferably comprises a check valve 74 which is connected to a source of high pressure air in the engine (not shown). The check valve closes when the pressure in the accumulator reaches approximately 300 psi and this pressure is maintained in accumulator 71. The pressure in accumulator 71 serves to raise the boiling temperature of the coolant used in the coolant circuit 18.

The coolant can comprise water or an aqueous solution of water and an antifreezing agent, such as ethylene glycol. However, in the event that the airstream directed through the second heat exchanger 21 is to later be directed to the cabin or cockpit of the aircraft, a leak in the second heat exchanger 21, which would allow ethylene glycol to enter this air, would be undesirable because of the toxic properties of this antifreeze; in this case, a low toxicity antifreeze, such as one used in commercial food refrigeration applications, would be used.

It is contemplated that the amount of heat needed to be disposed from the transmission 3 is in the approximate range of 3000 to 15,000 btu's per minute. Under these circumstances, it is considered that the total quantity of coolant contained in the coolant circuit will equal about 3 gallons. It is contemplated that other sources of oil beside the transmission 3 can be used to heat the coolant. One such source would be the lubricating oil of the engine 6.

An invention has been described which reduces two causes of wasted energy in turboprop propulsion systems: first, at least some of the heat produced by the transmission and otherwise wasted is returned to the engine cycle through preheating fuel and, second, the thrust-producing airstream otherwise diverted to cool the transmission is now made available for the thrusting function. Thus, the necessity of the transmission oil cooler 109 in Fig. 1 is eliminated except in the unusual case of the fuel providing insufficient heat sinking capacity. For such an unusual case, a standby oil cooler can be provided, and it will be positioned out of the airstream 24 of Fig. 2 except during use.

Further, fuel tank heating is accomplished without routing a hot engine oil to the fuel tanks and without routing fuel to the engine.

While one embodiment of the invention has been disclosed, it will be obvious to those skilled in the art that numerous substitutions and modifications can be undertaken without departing from the true spirit and scope of the present invention. In particular, it is foreseen that the duct 27 and the throttling valve 30 may be combined into a positionable duct which iself acts as a throttling valve. Also, it is foreseen that the sequence in which the coolant passes through the heat exchangers can be changed.

As an alternative to tapping the airstream 24 to cool the coolant circuit, tapping of a compressor bleed source in the turbine engine has been discussed. Of course, different stages in the compressor provide air at different temperatures and pressures, so that air of a proper temperature must be selected. Furthermore, it is possible to tap a compressor stage containing high temperature air in order to transfer heat into the coolant circuit. This may be desirable when the transmission oil provides insufficient heat for purposes such as fuel tank heating.

The use of a third temperature transducer 88 has been described to sense the temperature of the coolant circuit and, if the temperature is too high, the control 93 responds to, in effect, draw the inference that the fuel has insufficient heat sinking capacity to absorb transmission oil heat. There are numerous other ways to determine whether sufficient heat sinking capacity exists.

Accordingly, what is desired to be secured by Letters Patent is the invention as defined in the following claims.

Claims

CLAIMS What is claimed is:

1. A system for recovering waste heat in a lubricating oil in a fuel burning engine supplied by fuel from fuel tanks, comprising: (a) a coolant circuit for circulating a liquid coolant, (b) a heat exchanger for thermally connecting the coolant circuit with the lubricating oil, and (c) a heat exchanger for thermally connecting the coolant circuit with engine fuel for heat exchange therebetween.

2. A system according to claim 1 in which the engine fuel of (c) is en route to be burned in the engine.

3. A system according to claim 1 in which the engine fuel of (c) is withdrawn from and returned to fuel tanks.

4. A system according to claim 3 and further comprising a heat exchanger for thermally connecting the coolant circuit with engine fuel for preheating fuel en route to be burned.

5. A system according to claim 1 and further comprising means to control the amount of heat transferred from the coolant circuit to the engine fuel.

6. A transmission oil cooling system for use in a liquid fuel-burning turboprop engine having a compressor which drives a propeller, comprising: (a) a coolant circuit, including a pump, for circulating a liquid coolant; (b) a first heat exchanger means for connecting the coolant circuit with the transmission oil for heat transfer into the coolant; (c) pressurizing means for maintaining the coolant at a pressure sufficient to inhibit boil ing due to the heat transfer of (b); (d) a second heat exchanger means for selectively connecting the coolant circuit with an airstream for heat transfer into the airstream; (e) a third heat exchanger means for selectively connecting the coolant circuit with relatively high pressure fuel for heat transfer into the fuel; and (f) a fourth heat exchanger means for selectively connecting the coolant circuit with relatively low pressure fuel which is withdrawn from, and then returned to, a fuel supply for heat transfer to the fuel supply.

7. A system according to claim 6 in which the airstream of (d) is provided by a static bleed of the engine's compressor.

8. A cooling system according to claim 6 and further comprising: (g) first temperature sensing means for sensing the temperature of the high pressure fuel of (e) and (h) first control means coupled to the first temperature sensing means for reducing the heat transferred into the high pressure fuel of (e) when the temperature of (g) reaches a predetermined threshold.

9. A cooling system according to claim 8 and further comprising: (i) second temperature sensing means for sensing the temperature of the fuel supply of (f) and (j) second control means coupled to the second temperature sensing means for reducing the heat transferred into the fuel supply of (f) when the temperature of (i) reaches a predetermined threshold.

10. A cooling system according to claim 9 and further comprising: (k) sensing means for sensing when the heat sinking capacity of at least some of the fuel reaches a predetermined level and (I) third control means for diverting an airstream from the propeller for cooling the transmission only when the predetermined level of (k) has been reached.

11. A method of returning heat to the engine cycle in a system including a fuelburning turboprop engine supplied with fuel from tanks which engine drives a propeller through an oil-containing transmission, comprising the steps of: (a) transferring heat from the oil of the transmission to a liquid coolant circuit and (b) transferring heat from the coolant circuit to fuel en route to be burned.

1 2. A method according to claim 11 and further comprising the step of transferring heat from the coolant circuit to the fuel in the tanks.

1 3. A method according to claim 11 and further comprising the steps of sensing the temperature of the fuel en route to be burned and, if the temperature exceeds a predetermined threshold, of reducing the heat transfer from the coolant circuit to the fuel en route to be burned.

14. A method according to claim 1 2 and further comprising the steps of sensing the temperature of the fuel in the tanks and, if this temperature exceeds a predetermined threshold, of reducing the heat transfer from the coolant circuit to the fuel in the tank.

1 5. A method according to claim 11 and further comprising the steps of sensing the heat sinking capacity of at least some of the fuel and in diverting an airstream from the propeller for cooling the oil of the transmission only if the heat sinking capacity of this fuel reaches a predetermined level.

16. A method of returning heat to the engine cycle in a system including a fuelburning turboprop engine substantially as hereinbefore described with reference to Fig.

2 of the drawings.

1 7. A waste heat recovery system substantially as hereinbefore described with reference to and as illustrated in the drawings.