Shopping for a new heat exchanger? You have come to
the right place. Here you can learn about the different types of
exchangers and then order one by
calling one of the many reviewed manufacturers and retailers of heat
exchangers.
The Fundamentals of Heat Exchangers
Fluid flow
Inside a heat exchanger, the fluid flow is either tumultuous or laminar.
Turbulent flow produces better heat transfer, for the reason that it
mixes the fluid. Laminar-flow heat transfer relies completely on the
thermal conductivity of the fluid to transfer heat from inside a stream
to a heat-exchanger wall. An exchanger’s fluid flow can be determined
from its Reynolds number (Nre). If the Reynolds number is less than
2,000, the fluid flow will be laminar if the Reynolds number is greater
than 6,000, the fluid flow will be fully turbulent. The transition
region between laminar and turbulent flow produces rapidly increasing
thermal performance as the Reynolds number increases.
The type of flow determines how much pressure a fluid loses as it moves
through a heat exchanger. This is important since higher-pressure drops
require more pumping power. However a manufacturer will generally find
out the pressure drop, it is useful to predict the pressure drops that
can come about with changing rates of flow. Laminar flow produces the
smallest loss, which increases linearly with flow velocity. For
instance, doubling the flow velocity doubles the pressure loss. For
Reynolds numbers beyond the laminar region, the pressure loss is a
function of flow velocity raised to a power in the range 1.6–2.0. In
other words, doubling the flow could increase the pressure loss by a
factor of four.
Balance and effectiveness
The characteristics of fluids contribute to a basic property of heat
exchangers—the heat-transfer rate. The heat transferred to the colder
fluid must equal that transferred from the hotter fluid. So the heat
transferred per unit time equals the product of mass flow per unit time,
specific heat, and the temperature change. This quick calculation should
be done before specifying any heat exchanger. Although heat exchangers
are normally specified only with desired temperatures, m QV
Plate heat exchangers have high heat-transfer coefficients and area. The heat-transfer rate is the main criterion. An exchanger’s effectiveness is the ratio of the actual heat transferred to the heat that could be transferred by an exchanger of infinite size. Efficiency is the best way to compare different types of heat exchangers. For example, a hot fluid stream being cooled by a cold-fluid stream in a counter flow heat exchanger. When the hot stream exits the exchanger, it must be warmer than the inlet temperature of the cold stream. In an ideal heat exchanger, the outgoing hot stream’s temperature equals the incoming cold stream’s temperature. Besides, this heat exchanger’s cold stream exits at a temperature lower than the inlet temperature of the hot stream. Given that the temperature drop on the hot stream is greater than the temperature gain in the cold stream in this example, the product of the mass-flow rate and the specific heat of the hot stream must be less than that of the cold stream, because of the required heat-transfer rate balance.