Page 203 - Improving Machinery Reliability

174 Improving Machinery Reliability

However, we are certainly not ready to give a blanket endorsement to all fan-
cooled bearing housings. Air-cooled bearing housings can cause bearing distress by
allowing uneven cooling of bearing rings. The inner ring is mounted on the shaft and
the combined assembly represents a poor heat sink. In comparison, the bearing hous-
ing is a relatively effective heat sink, especially if cooled externally. The heat transfer
rate is influenced by the properties of the housing material, by housing geometry and
by the temperature difference between pumpage and external ambient conditions. Fan
cooling is forced convection and thus increases heat transfer. If inaterial properties
and housing geometry are assumed constant, cooling the housing by any of several
possible methods will increase the rate of heat transfer and thus cools the bearing
outer rings. Very little heat is transferred through rotating elements from the inner
ring. The inner ring, therefore, runs hotter than the outer ring.
The result, of course, is differential thermal growth, with the inner ring expanding
more than the outer ring. If bearings are initially flush ground (Le., no preload and
no end-play) or ground for preload, cooling the housing creates or, respectively,
increases radial and axial preload and negative clearance exists. The resulting tem-
perature excursion may or may not be self-limiting. In any event, there would now
be increased demands on the lubricant to effectively prevent metal-to-metal contact.
(See also Figure 3-47).

Stuffing Box Cooling Is Not Usually Effective
Many pumping services require that the mechanical seal environment be kept at
moderate temperatures. This is generally not difficult to achieve if external flush
injection is used. In this case, a flush cooler can perform the task, but, of course, at
some utility expense (fan power in air-cooled systems, cooling water in conventional
heat exchanger circuits). The cost of recirculating the flush fluid may have to be
added as well.
Another option for achieving a moderate seal environment is stuffing box cooling.
In conventional pumps, Figure 3-66, the stuffing box cavity (“A”) is rather remote
from the seal faces (“B”) that we wish to cool. An experiment conducted around
1970 showed a disappointing 1” to 2°F decrease at the seal faces when cooling water
was introduced into a previously empty stuffing box jacket.
A superior design, from the point of view of effective cooling, is shown in Figure
3-67. The manufacturer recognized that heat migration from the casing is primarily
responsible for elevated stuffing box temperatures. He, therefore, designed the pump
with an air gap “A” ahead of the cooling water cavity “B.” Equally important is the
fact that the throat bushing “C” is made extremely long and that cavity “B” contacts
the throat bushing over a good portion of this length. It should be intuitively evident,
however, that this configuration will lead to directionally higher L/D ratios than
competing designs. Accordingly, shaft deflections must be compensated by control-
ling the forces acting radially on pump impellers and by more careful design of com-
ponent clearances.