Power Plant Engineering

OB: In this method the cooling of an ON-type transformer is improved by air blast over the
outside of the tank.
OFN: The oil is circulated by pump to natural air coolers.
OFB: For large transformers artificial cooling may be used. The OFB method comprises a forced
circulation of the oil to a refrigerator, where it is cooled by air-blast.
OW: An oil-immersed transformer of this type is cooled by the circulation of water in cooling
tubes situated at the top of the tank but below oil-level.
OFW: Similar to OFB, except that the refrigerator employs water instead of air blast for cooling
the oil, which is circulated by pump from the transformer to the cooler.
12.4.2 MIXED COOLING
ON/OB: As ON, but with alternative additional air-blast cooling. ON/OFN, ON/OFB, ON/OFW,
ON/OB/OFB, ON/OW/OFW: Alternative cooling conditions in accordance with the methods indicated.
A transformer may have two or three ratings when more than one method of cooling is provided.
For an ON/OB arrangement these ratings are approximately in the ratio 1/1.5; for ON/OB/OFB in the
ratio 1/1.5/2.
12.4.3 NATURAL OIL COOLING
The diagram in Fig. 12.24 is drawn to indicate on the left the thermal flow of oil in a transformer
tank. The oil in the ducts, and at the surface; of the coils and cores, takes tip heat by conduction, and
rises cool oil from the bottom of the tank rising to take its place. A continuous circulation of oil is com-
pleted by the heated oil flowing to the tank sides (where cooling to the ambient air occurs) and falling
again to the bottom of the tank. Oil has a large coefficient of volume expansion with increase of tem-
perature, and a substantial circulation is readily obtained so long as the cooling ducts in the cores and
coils are not unduly restricted.
Fig. 12.24 also shows on the right a curve typical of approximate temperature distribution, the
figures quoted being rises in degrees centigrade. On full load with continuous operation, the greatest
temperature-rise wills probably he in the coils. The maximum oil temperature may be about 10° less
than the coil figure, and the mean oil temperature another 15° less.
The best dissipater of external heat is a plain blacked tank. But to dissipate the loss in a large
transformer a plain tank would have an excessively large surface area and cubic capacity, and would
require a great quantity of oil. Both space and oil are expensive.

---

404
POWER PLANT ENGINEERING
Artificial means for increasing the surface
area without increasing the cubical contents have,
therefore, been developed. These comprise special
tank constructions such as
(a) Fins, welded vertically to the tank sides;
(b) Corrugations;
(c) Round- or elliptical-section tubes;
(d) Auxiliary radiator tanks.
These are illustrated in Fig. 12.25. Little
need be said of (a) or (b), the former being not
very effective and the latter rather difficult in con-
struction, although formerly used in Europe.
Method (c) is extremely common for a wide range
of sizes, while (d) is used when there is insuffi-
cient room to accommodate all the tubes required
by a large transformer.
The tabbed tank provides considerable cooling surface, and the tubes being connected with the
tank at the top and bottom only provide a head sufficient to generate a syphoning action; which im-
proves the oil circulation quite apart from enhanced cooling. The tubes may sometimes be "gilled," i.e.
wound with a strip-on-edge metal helix, to increase cooling surface and the eddying airflow that more
effectively removes heat. The baffling action, however, also tends to restrict the total sir flow so that the
net gain is not commensurate with the added cooling surface.
Ribs or Fins
Oil
Tank wall
Corrugations
Oil
( )
a
( )
b
Tubes
( )
c
( )
d
Radiator
tank
Main tank
Ambient temp
Co
re
Du
c
t
L.
V
.
Du
c
t
H.
V
.
Ai
r
T
em
per
at
ur
e-
ri
s
e,
°
C
70
60
50
40
30
20
10
0