System efficiencies and space requirements remain the priorities in both large and small refrigeration plants. Semiwelded plate heat exchangers can address these priorities in a number of process cooling applications.

Plate heat exchanger technology has been utilized in the dairy and chemical industries since the late 1940s and early 1950s. But, it was not until the 1970s, when the demand for more energy-efficient products grew, that new developments in plate heat exchangers began. Several new plate designs and stamping techniques were developed, including the chevron plate design, which is a stamped plate with embossed ridges at high and low angles to increase the heat transfer effect.

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.

A semiwelded plate heat exchanger consists of a series of plate cassettes, end plates, plate seals, through-bolts and a carrying bar. To form the exchanger, a series of plate packs, which are two plates welded on the perimeter to create a welded plate pair or cassette, are arranged to provide separate flow paths, or channels, for refrigerant and liquid flow. These welded plate pairs are supported by the carrying bar and contained by pressure plates at each end (figure 1).

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 molded pattern pressed into the plates forms narrow flow-channels in which intense turbulence occurs. The continuous turbulence of the flow media results in high heat transfer coefficients and increases the temperature exchange.

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 Operation

Semiwelded 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.

In thermosiphon evaporators, the liquid refrigerant vaporizes in an expansion valve prior to entering a surge drum (figure 3). The liquid refrigerant from the surge drum flows by gravity to the evaporator, where it partially vaporizes. This two-phase mixture returns to the surge drum, where the vapor goes back to the compressor and the liquid recirculates back to the evaporator.

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.

In the pumped liquid overfeed system, the liquid refrigerant is pumped from a low pressure receiver to the evaporator at a specific flow rate (figure 4). The liquid and gas refrigerant mix returning from the evaporator re-enters the low pressure receiver and is removed through the suction line. Cocurrent process liquid flow is recommended. This type offers design flexibility for location of vessels and fittings.

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.

Links

• Paul Mueller Co.