Page 30 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

26 Chapter 2 Heat transfer processes in industrial scale

the fluidised side compared to particle free or dilute phase (particle laden) gas flow. In coal-fired
fluidised bed combustors, the fluid bed exchanger is an efficient way of heat extraction from the
bed. Typical applications of fluidised bed heat exchanger are drying, mixing, adsorption, reaction, coal
combustion and waste heat recovery.

2.2.4 Direct contact heat exchanger
In a direct contact heat exchanger, the hot and the cold fluids come in direct contact and exchange heat.
Most direct contact heat exchangers involve mass transfer in addition to heat transfer as in evaporative
cooling (cooling tower), cooling of hot gases with water spray, barometric condenser, etc. The
enthalpy of phase change is usually predominant in such an exchange. Direct contact exchangers are
advantageous due to (A) very high volumetric heat transfer rates, (B) inexpensive construction due to
minimal hardware requirement, (C) absence of heat transfer surface between the fluids and (D) no
fouling. However, these can be used only for services where direct contact between the streams is
allowable. Cooling tower discussed in Chapter 7 involves such direct contact heat transfer.

2.3 Flow arrangement
Each fluid in the exchanger may flow in single or multiple pass. A fluid makes one pass if it flows once
through the full length of a section of the heat exchanger. If the fluid subsequently reverses its flow
direction after full length flow and flows again through the same section, it makes a second pass.
Multipass arrangements in shell and tube exchangers are elaborated in Chapter 4.

2.3.1 Countercurrent flow exchanger
For single-pass exchangers, the flow of the fluids is usually co-current (parallel to each other in the
same direction) or countercurrent (parallel to each other but in the opposite direction). A higher
effective temperature difference results in case of countercurrent flow for the same inlet temperature of
the fluids and heat duty of the exchanger. This leads to lower heat transfer area and a smaller exchanger
as long as the overall heat transfer coefficient is nearly the same. In addition, the maximum tem-
perature difference across the exchanger tube wall either at the hot or the cold fluid end (for an
equivalent performance) is the lowest and that produces lower thermal stress in countercurrent flow as
compared to the other flow arrangements. There are manufacturing difficulties associated with true
counterflow arrangement in plate fin exchangers.

2.3.2 Co-current flow/parallel flow exchanger
Co-current flow has the lowest exchanger effectiveness among single-pass exchangers for a given
overall thermal conductance (U A e explained later), fluid flow rates and temperatures. These are
preferred when the cold fluid has high viscosity. The entering cold fluid at lower temperature ex-
changes heat with the entering hot stream at high temperature and gets heated quickly. This reduces its
viscosity and improves the cold fluideside heat transfer coefficient. This also reduces the pressure
drop for the cold fluid. However, the thermal stress in the exchanger at the inlet is higher due to the
higher temperature difference between the fluids.