Plate Heat Exchangers.
in Combined Heating and Cooling Systems
As a result of modern architecture and evolving comfort standards, buildings such as commercial buildings, hotels, hospitals, and mixed-use complexes no longer require only heating or only cooling. For a large part of the year, simultaneous heating and cooling demands can arise in different facades or different areas of the building. Designed to meet this complex thermal need, combined heating and cooling systems are one of the most sophisticated areas of HVAC (Heating, Ventilation, and Air Conditioning) engineering. The key component that ensures the efficient, safe, and flexible operation of these systems is the plate heat exchanger, which often remains in the background but has a critical function.
In this article, we will delve into the technical role of this engineering marvel at the heart of combined systems, its contribution to energy efficiency, and the correct integration scenarios.
General Structure of Combined Heating and Cooling Systems
Combined heating and cooling systems are HVAC systems that can provide both heated and cooled water to a building simultaneously. This allows an office on the north side to be heated while a meeting room or server room under heavy human and equipment load can be cooled simultaneously. The most common and well-known configuration is the “4-pipe system”.
The general structure of these systems consists of the following main components:
Heat Generation Center: Usually consists of condensing boilers, heat pumps, or a central heat source. This center produces hot water for the heating circuit.
Cooling Generation Center: Water is generated by chiller units. This center produces chilled water for the cooling circuit.
Distribution Network: These are the pipelines that transport heated and chilled water to the end-use points in the building. In a 4-pipe system, two pipes are used for hot water supply and return, and two pipes are used for chilled water supply and return.
End-Use Equipment: These are equipment such as fan coil units (FCUs) and air handling units (AHUs). These units have both hot and cold water coils and heat or cool the environment by opening the relevant valve according to the thermostat command.
Hydraulic Separation and Control Equipment: Pumps, valves, sensors, and plate heat exchangers that form the backbone of this system.
The Role and Advantages of Plate Heat Exchangers in These Systems
Plate heat exchangers play multiple critical roles in these complex systems. Their primary function is to physically separate the primary circuit, where the heating and cooling sources are located, from the secondary circuit, where this energy is distributed to the building. This “hydraulic separation” principle provides numerous engineering advantages:
Equipment Protection: Chillers and boilers are the most expensive and sensitive components of the system. The plate heat exchanger acts as a barrier, preventing potential contaminants, sediment, air, and chemicals that may be present in the water circulating in the building’s distribution network from reaching this expensive equipment.
Fluid Compatibility: In cooling circuits, especially in pipelines operating in outdoor environments, it may be necessary to add glycol to the water to prevent freezing. However, many chillers achieve their highest efficiency when operating with pure water. Plate heat exchangers make it possible to safely use pure water in the primary circuit (chiller side) and glycol-containing water in the secondary circuit (building side).
Pressure Reducing Function: Especially in high-rise buildings, significant pressure differences occur between lower and upper floors due to static pressure. Heat exchangers divide the building into different pressure zones, allowing each zone to operate in its own ideal pressure regime and preventing equipment from being exposed to high pressure.
System Flexibility and Modularity: The use of heat exchangers allows different parts of the system to be designed, operated, and maintained independently. Changes or maintenance in one circuit do not affect other circuits.
Thermal Equilibrium and Flow Direction Management
The thermal performance of plate heat exchangers is directly related to the way fluids pass through the plate surfaces. For maximum efficiency, the counter-current flow principle is used in almost all applications. In this principle, hot and cold fluids enter and exit the heat exchanger from opposite directions.
This arrangement ensures a more homogeneous distribution of the temperature difference (ΔT) across the heat exchanger, maximizing the Logarithmic Mean Temperature Difference (LMTD). A high LMTD means less plate surface area is needed for the same heat transfer, resulting in a more compact and economical heat exchanger.
One of the most critical parameters in the design phase is the approach temperature. This is the difference between the outlet temperature of one circuit and the inlet temperature of another. For example, in a cooling circuit, if water from the chiller enters at 7°C, returns through the heat exchanger at 12°C, and is sent to the building circuit at 9°C, the approach temperature is (9°C – 7°C) 2°C. Lower approach temperatures mean higher efficiency, but also require more
