Most shell-and-tube heat exchangers for HVAC&R applications are counter flow. The fluid at temperature T1, enters one end of the shell, flows outside the tubes and inside the shell, and exits at the other end at temperature T2. The other fluid flows inside the tubes, entering one end at temperature Ta and exiting at the opposite end at temperature Tb.
Shell-and-tube heat exchanger can use steam or hot water as primary fluid.
