It was developments in the 1990s, including creation of the mixed chevron pattern on a single plate and modification of the welded plates, that attracted the industrial refrigeration industry. These developments led to semiwelded plate heat exchangers being employed in many refrigeration duties, including ammonia applications. Refrigeration plants found that the mixed chevron design tends to reduce flow mal-distribution, self-adjusts to process stream needs and optimizes the heat transfer area.
By contrast, a typical shell-and-tube heat exchanger consists of tube bundle within a shell. The shell-and-tube heat exchanger transfers heat from the tube bundle to the shell, or from the shell to the tube bundle. To achieve the results required, shell-and-tube heat exchangers rely on high volumes of refrigerant passing between the tube bundle and the shell.
Due to its compact modular design, the installation space and weight of the semiwelded plate heat exchanger can be less than that of a comparable shell-and-tube ex-changer. Standard components primarily are bolted together, which facilitates future capacity increases. The adjustable through-bolt connection eases inspection and expansion.
Seal and plate materials are selected after taking into account the temperatures and corrosive properties of the media involved. Typical seal materials include Nitril, EPDM, Viton, Neoprene and Chloroprene; plate materials typically are AISI Type 304 or 316 stainless steel and titanium. Other benefits of the semiwelded plate heat exchanger include:
- Plates can be cleaned on the sealed side.
- Seals can be changed during normal maintenance.
- Individual plate cassettes can be exchanged when required.
The small space within each plate pack typically leads to reduced volumes of refrigerant in the evaporator vs. most other types of evaporators. Due to the turbulent flow created in the plate gap, particles of dirt and impurities are kept in a mixed state longer than in conventional heat exchangers. High shearing forces on the smooth, heat transfer wall minimize the deposition of fouling layers and thus generate a self-cleaning effect.
Evaporator OperationSemiwelded plate heat exchangers are designed to operate with a range of refrigerants in direct expansion, thermosiphon (flooded) and pumped liquid overfeed systems. For direct expansion evaporators, the refrigerant cools down and partly vaporizes as it enters the thermostatic expansion valve (figure 2). A sensing bulb at the exit of the evaporator controls the valve via a superheat setting. As refrigerant enters the evaporator in the form of a two-phase mixture, it evaporates completely within the evaporator to the point of superheating.
One concern with this approach is if oil is not purged properly, it can begin to fill the evaporator, greatly reducing the system's thermal efficiency. Process liquid should enter the evaporator in a cocurrent flow as this ensures that the initial temperature difference is substantial enough to assist in promoting refrigerant boiling. A level device maintains a constant level of liquid refrigerant in the surge drum. It is important to keep enough liquid leg to maintain a positive flow.
Due to the design of semiwelded plate heat exchangers, less refrigerant is required at the evaporator. This feature, along with the vertical design, permits using smaller vessels and less complex piping.