2022年3月22日
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MKP ENGINEERING LLC
Eye opening Engineering 《No.2》Second law of thermodynamics – Heat move from high temperature part to low temperature part. –
Eye opening Engineering 《No.2》Second law of thermodynamics – Heat move from high temperature part to low temperature part. – – Heat move from high temperature part to low temperature part. –
This law is widely known to everyone.
Let me introduce one example of plant engineering in accordance with this law.
On engine operation, Lube oil is essential to be used for lubrication to rotation part and rubbing part of reciprocation in order to operate smoothly. In addition, cooling water is used for cooling of high temperature parts due to heat by combustion of fuel oil or fuel gas inside cylinder while engine make rotational force to crankshaft with pushing piston down by combustion in combustion chamber.
The outlet temperature of this cooling water on engine becomes higher than the inlet temperature because water get rid of heat by combustion during operation. This heat from engine is released to atmosphere by using auxiliary equipment such as cooling tower or radiator, etc.
Nevertheless, lower water temperature is not better, adequate temperature is required. The water at engine outlet is controlled as about 90 degree C so as to cooling and keeping adequately for combustion chamber.
Lube oil has also function of neutralization of acid generated during combustion of heavy fuel oil besides above mentioned lubrication.
Moreover, Lube oil temperature increase due to heat of combustion same as cooling water. By using Lube oil cooler, the heat is transferred to cooing water and released to out of system. Lube oil has such cooling performance.
Lube oil at engine inlet is controlled as about 60 degree C in consideration of film thickness at bearings and friction of rubbing parts.
During operation of engine, Lube oil and cooling water is controlled as above mentioned way. But during stoppage of engine, temperature of Lube oil and cooling water is the same level of atmosphere.
When engine start after long-term stoppage, it will be no problem in spring, summer and autumn because temperature of Lube oil and cooling water is not so cold. However, in winter there may happen some trouble due to very cold Lube oil and cooling water at engine start.
The following is an example in the power plant in which I worked as technical advisor.
The operation of this power station is mainly continuous use (*1) except for the stoppage during maintenance period.
Under this kind of operation, there is no problem due to no chance of very low temperature of cooling water and Lube oil.
(*1) In case of emergency use of power station, there are heaters in cooling water line and Lube oil line. By means of these heaters, even at stoppage of engine, the cooling water line and Lube oil line is warm condition so as to enable engine start immediately at emergency case.
But in case of continuous use of power station, there are no such heaters.
But unfortunately, fuel cost increased drastically at one point, operation made going into the red. As a result, engine operation was suspended. During long-term stoppage of engine, engine manufacturer recommended to operate the engine and circulate Lube oil into inside of engine periodically. Then it was decided that engine started once per two weeks.
In winter, especially in January and February, the starting condition is very severe that atmospheric temperature becomes nearly zero degree C, and temperature of cooling water is less than 20 degree C.(*2) Lube oil is heated by electric heater in the purifier unit. But the temperature of Lube oil is around middle of 20 degree C level (*2) because of heat radiation from piping, etc.
(*2) Since engine is installed in power house, temperature of cooling water and Lube oil is certain level, even atmospheric temperature is zero.
Under such conditions, it is not recommendable to start engine because of high risk.
When Lube oil temperature is low, viscosity becomes high and friction loss increases at bearing part.
On starting engine, high pressure air is injected into combustion chamber, i.e. piston, having adequate timing. Then piston goes down and crankshaft rotates. But rotation speed becomes slow under such conditions.
As a result, the ignition inside combustion chamber may be missed and starting failure may happen.
In order to overcome such condition, I considered how to increase the temperature of Lube oil before starting.
Finally, I found that cooling water temperature is increased by steam and the heat of cooling water is transferred into Lube oil at inside of engine. The injection of steam into cooling water is adopted as a countermeasure of anti-freezing of cooling water pipe and radiator located outdoor.
At that time, since water temperature is more than zero degree C so as not to freeze, injection volume of steam is not so much.
But in order to transfer heat from cooling water to Lube oil, temperature of cooling water is required around 50 to 60 degree C.
The available steam is 7 kg/cm2g saturated, 169 degree C temperature.
Then I checked the seat material of valves near steam injection point and confirmed it as high temperature proof material.
On the starting day after long-term stoppage, injection of steam into cooling water is started around 8:00 o’clock in the morning and continued until around noon. At last, the cooling water temperature reaches the above-mentioned level. In parallel Lube oil temperature also goes up and reaches above 30 degree C in the afternoon.
At this condition of Lube oil temperature, it is possible to start engine. But the temperature level seems still low.
The starting sequence is as below.
- Engine start and speed increase up to 600 rpm.
- Engine speed at 600rpm is kept for 5 minutes.
- Then speed go up to rated speed of 750rpm.
In view of my experience, engine starting is possible at 30 degree C of Lube oil temperature. During engine speed goes up from 600rpm to 750rpm, inlet pressure at Lube oil main filter goes up rapidly and it becomes a dangerous level. (*3)
(*3) The supply of Lube oil into engine is done by Lube oil priming pump driven with motor before and during starting of engine.
After starting, the supply is done by Lube oil pump integrated in engine.
This Lube oil pump is driven by use of rotation of crankshaft. Therefore, the rotation speed changes in accordance with engine speed. As a result, supply pressure of Lube oil also changes.
The supply pressure is different from each other at 600rpm and 750rpm. It affects to the inlet pressure of Lube oil main filter.
In addition, the pressure is related with temperature which influences
viscosity of Lube oil.
Then as another trial, after the temperature of Lube oil became 40 degree C with more heating time, engine was started. It was smooth starting. However, it took so much time.
In order to resolve above matters, I found the following method.
The period of 5 minutes for keeping 600 rpm will be extended to 10 minutes.
By means of this method for extension of operation at 600 rpm, temperature of Lube oil increase and pressure go down because of lower viscosity.
As a result, even when temperature of Lube oil is 30 degree C, engine can be started without excessive high pressure at inlet of Lube oil main filter.
Over
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