Turbine anti-icing system

Exhaust gas is recirculated from the exhaust stack of a gas fired turbine to the air inlet along a constantly-open path to prevent inlet freeze-up. When anti-icing is not needed the exhaust stack is fully opened, creating a partial vacuum in the exhaust stack. At the turbine inlet the recirculation line, is opened to atmosphere. The resultant pressure differential between the opposite ends of the recirculation line creates a driving force for positively purging the recirculation line of unwanted residual exhaust gases. This in turn eliminates a source of unwanted moisture which could otherwise condense, freeze and interfere with turbine operations.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention is related generally to the field of gas fired turbines and 
more particularly to apparatus for prevention of icing in such turbines in 
the presence of low ambient temperatures. 
2. Prior Art 
A gas fired turbine draws in fresh air, which is compressed, and then fed 
into a combustion chamber which provides combustion gas to drive turbine 
blades. In a cold environment, when ambient temperatures drop below 
+35.degree. F., the air inlet to the turbine may freeze up due to 
condensation of water vapor in the air, blowing snow or ice fog. To 
overcome this problem, some means of anti-icing are essential. In 
accordance with a typical prior art anti-icing system, a recirculation 
line interconnecting the air inlet and the exhaust gas stack permits 
enough hot exhaust gases to be constantly recirculated to the inlet to 
maintain turbine inlet air temperature at a minimum of +40.degree. F. A 
bypass valve is positioned in the exhaust stack channel and dampers are 
provided which can close off the recirculation line. When anti-icing is 
not needed, the recirculation dampers are closed and the exhaust bypass 
valve is opened. When anti-icing is needed, the recirculation dampers are 
opened and the exhaust bypass valve is closed sufficiently to create a 
back-pressure which forces exhaust gases back to the turbine inlet. 
The problem with this prior art system is that when anti-icing is not 
required, the exhaust gas is allowed to occupy the recirculation line 
between the exhaust stack and the recirculation dampers. As a result, 
moisture in the exhaust gas condenses out and accumulates around the 
recirculation line dampers, causing the dampers to freeze in a closed 
position. The result is that the dampers are inoperable when needed. To 
avoid this possibility, gas turbine operators have been forced to 
constantly recirculate enough exhaust gas to maintain the inlet air 
temperature at +40.degree. F. so as to prevent the recirculation dampers 
from freezing. This solution has other drawbacks, including loss of 
horsepower, constant recirculation of corrosive exhaust gas and a hotter 
hot-gas path in the turbine. Furthermore, with constant recirculation of 
exhaust gases, so much moisture is present at the air inlet that when 
ambient temperature drops below -40.degree. F., condensation may cause the 
turbine to freeze up on the inlet unless constant attention is devoted to 
avoidance of this condition. 
SUMMARY OF THE INVENTION 
The general object of this invention is therefore to provide an improved 
anti-icing system for a gas fired turbine. A more particular object of 
this invention is to provide an improved anti-icing system for a gas fired 
turbine which is effective and reliable but avoids excessive use of 
exhaust gases for anti-icing purposes. 
In accordance with a preferred embodiment of this invention, an anti-icing 
system is disclosed for a gas fired turbine having an air inlet and an 
exhaust stack open to the atmosphere which are interconnected with a 
constantly open free-flowing recirculation line. An exhaust by-pass valve 
in the exhaust stack may be operated from fully opened to partially 
closed. In the partially-closed condition of such valve, sufficient 
back-pressure is created to force recirculation of exhaust gas to the 
turbine inlet through the recirculation line. When anti-icing is not 
necessary, the bypass valve is fully opened to create a partial vacuum 
within the exhaust stack. With the air inlet end of the recirculation line 
open to atmosphere a pressure-differential is created which causes air to 
be drawn from the recirculation line into the exhaust stack thus purging 
the recirculation line of unwanted residual exhaust gases. 
In the preferred embodiment, the recirculation line comprises a recycle 
manifold consisting of an open-ended recycle header interconnected with a 
plurality of parallel closed-end air distribution pipes each provided with 
vent holes facing the direction of inlet air. In an anti-icing operation, 
hot exhaust gas enters the distribution pipes at one end from the header 
and discharges through these vent holes to mix with slow-moving ambient 
inlet air which is then drawn into the turbine through a turbine filter or 
inlet screens. When anti-icing is not needed, purge air is drawn into the 
recycle header from the vent holes in the distribution pipes. 
Further objects and advantages of this invention will become apparent from 
a consideration of the detailed description to follow taken in conjunction 
with the appended drawings and claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
With reference now to FIG. 1, in a gas fired turbine anti-icing system 10 
in accordance with the prior art, inlet air 12 is drawn into turbine inlet 
air filter 14 to support gas combustion, while exhaust gases are expelled 
through turbine exhaust stack 16. An exhaust bypass valve 18 situated 
within exhaust stack 16 may be rotated so as to vary the number of degrees 
from a fully-open position indicated in dotted outline by the reference 
numeral 18 prime. Recirculation piping consisting of lines 22, 23 and 24 
provides a path from exhaust stack 16 to the air inlet to air filter 14. 
Recirculation damper valves 26 are inserted in lines 23 and 24. 
In an anti-icing operating, bypass valve 18 is partially closed and 
recirculation dampers 26 are opened to assume the dotted position 
indicated by the numeral 26 prime. Sufficient back pressure in exhaust 
stack 16 is then created to force a certain amount of exhaust gas along 
recirculation path 28 through lines 22, 23 and 24 and thus to the inlet to 
turbine air filter 14. When anti-icing is not required, bypass valve 18 is 
opened and dampers 26 are closed. Exhaust gas then proceeds exclusively 
along path 30. 
As previously noted, the problem with this arrangement is that residual 
exhaust gas in recirculation line 22 is not purged. As a result, the 
moisture in such gas tends to condense, accumulate around recirculation 
dampers 26 and then freeze dampers 26 in a closed position. When this 
happens, of course, anti-icing is not available when required. The only 
alternative to avoid this possibility is to maintain a constant 
recirculation of exhaust gas with its attendant disadvantages as 
explained. 
One reason for this problem is that prior art systems such as shown in FIG. 
1 typically include a mechanical stop (not shown) within the exhaust stack 
16 which prevents valve 18 from reaching fully open position 18 prime. The 
purpose for this stop is to prevent valve flutter. Tests show that 
depending upon the particular mechanical configuration of the components 
of a turbine anti-icing system and its operating condition, exhaust gas 
recirculates to the turbine inlet at some small bypass valve angle. FIG. 7 
illustrates this relationship, and shows that so long as valve 18 is 
stopped before reaching fully open position 18 prime complete purging of 
recirculation lines 22, 23 and 24 may not occur. For example the graph 
indicates that at some angle slightly in excess of 2.5 degrees turbine air 
inlet temperature starts to rise rapidly. However, even eliminating the 
mechanical stop and allowing bypass valve to operate in the fully open 
position 18 prime will not achieve the advantages of applicant's system as 
will be seen. 
In the flow diagram of FIG. 2, an improved anti-icing system 40 comprises 
turbine inlet air filter 42 adapted to receive ambient inlet air 44 and to 
expel exhaust gases 45 through exhaust stack 46. Exhaust gas recirculation 
means are provided consisting of line 48 interconnected with stack 46 at 
one end and at the other end with lines 50 and 52 whose ends 54 and 56 
adjacent the inlet to inlet filter 42 are open to the atmosphere. When 
anti-icing is required, exhaust bypass valve 60 is closed sufficiently to 
cause a back pressure which forces exhaust air into recirculation line 48 
along line 62 which passes unimpeded through lines 50 and 52 to 
intermingle with inlet air 44. In FIG. 3, the same system 40 is operated 
when anti-icing is not required. In this mode, bypass valve 60 is fully 
opened causing a partial vacuum within exhaust stack 46. This partial 
vacuum together with the atmospheric pressure at line ends 54 and 56 
creates a pressure differential which purges lines 48, 50 and 52 along 
path 64, thus eliminating therefrom residual exhaust gas with its 
attendant problems. 
A significant advantage of the arrangement described in FIG. 2 and FIG. 3 
is of course the elimination of the damper valve 26 as discussed in 
connection with the prior art system of FIG. 1 and thus of the possibility 
of the malfunction of such devices. The apparatus of FIG. 2 and FIG. 3 
also therefore eliminates the need for constant recirculation of exhaust 
gas to the inlet to the turbine. 
In the practice of this invention it is important to maintain full 
atmospheric pressure at the point in the recirculation lines such as ends 
54 and 56 where they open up at the inlet to filter 42. This maximizes the 
driving force so that under the conditions of FIG. 3 recirculation piping 
48, 50 and 52 is constantly purged with air. FIG. 4 illustrates one means 
for accomplishing this purpose. A recycle exhaust gas manifold 70 
comprises recycle header 72 interconnected at spaced-apart intervals with 
a plurality of closed-end distribution pipes 74 each of which is in turn 
provided with spaced-apart vent holes 76 facing the upstream direction 
from which inlet air flows. The ends 80 of distribution pipes 74 remote 
from header 72 are closed. When anti-icing is required exhaust gases enter 
header 72 at inlet 84, are distributed into pipes 74 along paths 86, and 
vented to atmosphere through holes 76, the opposite end 88 of header 72 
being closed. As best seen in the detail of FIG. 5, exhaust gas entering 
distribution pipes 74 from header 72 mixes with low velocity inlet air 92 
along paths 93 before striking turbine inlet screens 94. The direction of 
flow through header 72 is indicated by arrow 96. When anti-icing is not 
required ambient air enters header 72 along paths 98 through vent holes 76 
into pipes 74 and delivers purge air to the system along path 100 as seen 
in FIG. 6. 
In a practical design for manifold 70, distribution pipes 74 may, for 
example, be about 12" in diameter and vent holes 76 about 4" in diameter 
to insure that they will not freeze over. 
It should be understood that all parts of the distribution manifolding 70 
must be upstream of all screens, filters and enclosures to establish that 
header 72 is fully opened to the atmosphere as described. The holes 76 
should face the direction from which the inlet air is coming in order to 
take advantage of the velocity head. 
Another critical aspect of the system of this invention is that there be a 
vacuum inside the exhaust stack. It has been found, for example, that it 
is advantageous to achieve a vacuum greater than one inch of water inside 
the stack 46. In order to accomplish this, the diameter of stack 46 should 
be such as to allow a sufficiently high interior velocity to create such a 
vacuum. To this end the exhaust stack 46 must also have a low-pressure 
differential between its end vented to atmosphere and the point at which 
it is joined by recirculation line 48. For this purpose one should avoid 
making the stack too narrow which could create back pressures as a result 
of excessive wall friction. Another caution is that one should not install 
silencer baffles or any other obstructions in the exhaust stack 46 above 
the point of connection with recirculation line 48, except of course for 
the exhaust bypass valve 60 itself. Finally, the valve 60 should exhibit a 
low-pressure drop across itself in the fully opened position. It should be 
borne in mind also that valve 60 does not necessarily have to be the full 
diameter of exhaust stack 46. 
As previously noted, with increase of the valve 60 angle both inlet air 
temperatures and thus turbine operating temperatures rise rapidly. It is 
known that a small change in turbine air inlet temperature changes the 
exhaust temperature by approximately a 21/2 to 1 ratio and that therefore 
any increased recirculation of exhaust gas is reflected immediately with 
an increase in operating temperature. The conclusion is that one should 
design valve 60 so that one can control inlet air temperature for 
anti-icing without "overshoot", thereby avoid unnecessarily hot turbine 
operation. One such design is to make valve 60 tapered like a knife blade 
toward both ends and give it a total diameter less than that of the 
internal diameter of stack 60. The optimum configuration will depend upon 
the particular equipment used. With the system of this invention, because 
there is always an unobstructed path between exhaust stack 46 and air 
inlet 42, failure to set by pass valve 60 in a fully open position will be 
reflected quickly in an increased inlet air temperature. This indication 
is not achieved in any system which utilizes dampers in the recirculation 
path. Thus the system of this invention provides means for automatically 
warning of a potentially dangerous exhaust gas recirculation.