Heat exchanger in horizontal design for two fluids or two gases with nonexchangeable straight-tube bundle (also suitable for condensation within the shell). The use of double tube sheets ensures a safety space that in case of leakage prevents the contamination of tube attachments. In case of leakage the pressure in the safety space rises. Then a targeted safety switch-off can be initiated or a scheduled repair can be made. Depending on the operating conditions a compensator may be required to compensate for expansion.