Introduction
Plate and frame heat exchangers, commonly known as "plate" heat exchangers, originated over 60 years ago in the European food industry. There was a need for a heat transfer device that was energy efficient, compact, easy to clean, and capable of being modified as design conditions changed. These original requirements were met with a plate and frame heat exchanger. Today the same fundamental needs exist and plate heat exchangers are used worldwide in most industries.

The typical components of HVAC plate heat exchangers are: -Plates: 0.4-0.6 mm thick stainless steel or titanium in various corrugated patterns. -Gaskets: NBR or EPDM, for water or steam service, respectively. -Front and Rear Heads: Painted carbon steel frames that hold the plate pack in compression. -Guide Bars: Carbon steel or stainless rods that support the plates and keep them in alignment. -Compression Bolts: Plated steel bolts that compress the plates and frames. -Ports: Lap joint flanged connections or studded ports with rubber or stainless steel liners These components are assembled into custom designed exchangers that have significant advantages over shell and tube heat exchangers. First cost is usually less due to the overall economical design of the plate and frame heat exchanger. Performance is superior due to the counter-current turbulent flow that results in high heat transfer coefficients. Higher heat transfer coefficients require less heat transfer surface area. Surface area is very compact with corrugated plates. Close approach temperatures (1-2°F) can be achieved. Maintenance cost is reduced due to easy access and disassembly. Installation costs are less because of the compact size, reduced weight, and less space requirement. There are no tube bundles to pull and all maintenance is done within a narrow perimeter of the unit. Designs are flexible and can be modified after installation. Plates can be added or removed. Various plate patterns can be used to optimize the design based upon heat transfer duty and pressure drop. Single units can be designed to perform multiple duties with three or more fluids.

Design Details
Plate patterns are usually horizontal or vertical chevrons. Horizontal patterns have more restriction to flow which causes more turbulence, higher heat transfer rates, and more pressure drop. Vertical patterns have less restriction, less turbulence, and less pressure drop. Thermal performance and pressure drop can be optimized by using a combination of horizontal and vertical plates (see Figure 3). Dirty liquids or streams containing particles require a special "free-flow" type plate with wide gaps that allow the particles to pass through the exchanger.

Operating pressures need to be specified at bid request. The effects of operating pressure are more significant now that plates are getting larger. Most buildings are designed for 150 psig with an ASME code rating. But the operating systems in the building do not necessarily operate at 150 psig. The chiller and the cooling tower probably operate at slightly different pressures. The emphasis is on "different." Large exchanger plates flex in the middle and "cave in" toward the low pressure side, creating a slightly higher pressure drop due to the restricted flow channels. Correspondingly, the high pressure side expands the plate gap and pressure drop decreases. These differences are relative to the design of the exchanger and are typically in the 5% range; however, this can increase with unbalanced flows. For example, if the hot side has 2000 gpm and the cold side has 1200 gpm, the difference is magnified. These pressure drop differences are usually not a problem unless the pump has been designed with minimum excess pressure. One of the good things about a plate heat exchanger is that more plates can be added to relieve the pressure drop.

Fouling factors do not have the same relevance to plate heat exchangers because the plates cannot be lengthened to accommodate the fouling factor percentage. Instead of making the plates longer, more plates are added to get the extra heat transfer surface area. The addition of plates creates more "tubes" or flow paths between the plates. As a result, velocity, turbulence, and heat transfer rates are reduced. In addition, the slower velocities sometimes promote fouling via the settling of suspended particles. Plate heat exchangers work best at design flow rates with optimum surface area for the heat duty. This optimization keeps the velocity up and allows peak performance. If the operation changes to reduced flow rates, the exchanger will work better if plates are removed. Likewise, an increase in flow rate gives increased velocity, which will generally result in higher heat transfer rates with an increase in pressure drop. Plates can be added as required for duty or pressure drop. Excess surface area should be kept at a minimum and extra frame length should be specified for future expansion. Fouling factors should be left to the manufacturer.