Plate Type Heat Exchangers

Introduction

Heat exchanger is a device that transfers heat through a conducting wall from one fluid to another. Heat exchangers are used to transfer heat from a hotter fluid (liquid or gas) to a colder fluid. This broad definition covers a wide range of equipment, including boilers, condensers, distilling plants, and ventilation cooling coils.

There are many different names for heat exchangers because of their varied applications. When considering what you want from a heat exchanger you should think about which medium you want to heat or cool and what you are using it for.

The Different Types of Heat Exchangers

1. Finned Tube Heat Exchanger or Air Cooled Heat Exchanger
2. Shell and Tube Heat Exchanger
3. Plate Heat Exchanger or Gasket Plate Heat Exchanger

Finned Tube Heat Exchanger or Air Cooled Heat Exchanger

A finned tube heat exchanger, air cooled heat exchanger (ACHE), cooler or dryer works by flowing a liquid into a series of tubes, whilst pumping gas or air around or over the pipework to cool the fluid down. Sometimes finned tube heat exchangers, ACHEs, coolers or dryers are enclosed in duct work, and sometimes they are fully exposed with air flowing over them. The efficiency of these units comes from the extended surface – the fins – that protrude into the air/gas flow and improve the heat transfer ability of the structure.

Finned tube heat exchangers are often used for heat recovery in processes that exhaust hot gasses. The heat in the gas is transferred into a liquid, usually water or a thermal oil. The heated liquid can be then used in an application where you would normally use even more energy to heat it up. ACHEs are ideal for chemical applications, petrochemical cooling, steam cooling, in textiles processing, grain drying, concrete curing, paper manufacture and food processing. As air is the most used process fluid in the world, the application range for ACHEs is extremely varied.

Shell and Tube Heat Exchanger

Shell and tube heat exchangers work by passing a hot or cold fluid or gas through a series of tubes (known as a tube bundle) enclosed in a large metal shell. The counter flowing hot or cold fluid or gas is pumped into the shell – where the heat transfer occurs.

Typically, these designs are used for high pressure applications, but conversely also where a vacuum condition may require a structure that can cope with high stresses. The containment aspect of a shell and tube is such that it may be more suitable for hot gasses than a finned tube bank, particularly where the gases maybe noxious or dangerous to health, or mandated to be kept away from release into the atmosphere. Common applications of the shell and tube heat exchanger are within oil, gas and chemical industries.

Plate Heat Exchanger or Gasket Plate Heat Exchanger

Plate heat exchangers or gasket plate heat exchangers work by passing fluids through a series of plates that are compacted together side-by-side.

Plate heat exchangers are most often found in liquid to liquid applications, such as hot process water that contains chemicals/contaminants heating up cold mains water to provide clean hot water. District heating systems can benefit from plate heat exchangers or gasket plate heat exchangers, allowing individual houses to use the correct amount of hot water from a centralized source. Plate heat exchangers can also be used to cool oils using water, where the two liquids can’t mix.

The plate heat exchangers consist of five basic elements: the cover, the carrier rail, the heat transfer plates, the support column, and the tie bolts. The inlet and outlet for both fluids are usually located in the same cover. The fluids are separated by the heat transfer plates. Each plate contains a gasket that fits into grooves pressed in the plate and in the nozzle ports. The gasket prevents the two fluids from mixing. The gasket is vented to the atmosphere, which permits a leak to be promptly detected. The plates are sandwiched between a fixed cover (follower) and a movable cover (Head) by tie bolts. The top and bottom carrier rails (bars) align the plates to each other.

Thermal plates and gaskets

The plates, which are supported beneath and located at the top by parallel metal bars, are held together against an end plate by tie bolts. Four branch pipes on the end plates, align with ports in the plates through which two fluids pass. Seals around the ports are so arranged that one fluid flows in alternate passages between plates and the second fluid in the intervening passages, usually in opposite directions.

The most important and most expensive part of a PHE is its thermal plates, which are made of metal, metal alloy, or even special graphite materials, depending on the application. Stainless steel, titanium, nickel, aluminum, incoloy, hastelloy, monel, and tantalum are some examples commonly found in industrial applications.

The plates may be flat, but in most applications have corrugations that exert a strong influence on the thermal-hydraulic performance of the device. Some of the main types of plates are shown in below Figure 3, although the majority of modern PHEs employ chevron plate types. The channels formed between adjacent plates impose a swirling motion to the fluids, as can be seen in Figure 4. The chevron angle is reversed in adjacent sheets, so that when the plates are tightened, the corrugations provide numerous points of contact that support the equipment. The sealing of the plates is achieved by gaskets fitted at their ends. The gaskets are typically molded elastomers, selected based on their fluid compatibility and conditions of temperature and pressure. Multi-pass arrangements can be implemented, depending on the arrangement of the gaskets between the plates. Butyl or nitrile rubbers are the materials generally used in the manufacture of the gaskets.

Fig. 3: Typical categories of plate corrugations. (a) washboard, (b) zigzag, (c) chevron or herringbone, (d) protrusions and depressions (e) washboard with secondary corrugations, e (f) oblique washboard

Fig. 4: Turbulent flow in PHE channels

The plate corrugations promote turbulence in the flow of both fluids and so encourage efficient heat transfer. Turbulence as opposed to smooth flow causes more of the liquid passing between the plates to come into contact with them. It also breaks up the boundary layer of liquid which tends to adhere to the metal and act as a heat barrier when flow is slow.

The corrugations make the plates stiff so permitting the use of thin material. They additionally increase plate area. Both of these factors also contribute to heat exchange efficiency. Excess turbulence, which can result in erosion of the plate material, is avoided by using moderate flow rates. However, the surfaces of plates which are exposed to sea water are liable to corrosion/erosion and suitable materials must be selected.

Titanium plates although expensive, have the best resistance to corrosion/erosion. Stainless steel has also been used and other materials such as aluminum-brass. The latter may not be ideal for vessels which operate in and out of ports with polluted waters. The corrosion resistance of titanium has made it a valuable material for use in sea-water systems whether for static or fast flow conditions. The metal is light weight (density 4.5 kg/m3) and has good strength. It has a tolerance to fast liquid flow which is better than that of cupro-nickel. It is also resistant to Sulphide pollution in sea water.

While titanium has great corrosion resistance because it is more noble than other metals used in marine systems, it does tend to set up galvanic cells with them. The less noble metals will suffer wastage unless the possibility is reduced by careful choice of compatible materials, coating of the titanium, insulation or the use of Cathodic protection.

The use of nitrile or butyl gaskets and bonding with special glue is very common in such heat exchangers. Removal is facilitated with the use of liquid nitrogen which freezes, makes brittle and causes contraction of the rubber seal which is then easily broken away. Other methods of seal removal result in plate damage. Nitrile rubber is suitable for temperatures of up to about 110 C. At higher temperatures the rubber hardens and loses its elasticity.

The joints are squeezed when the plates are assembled and clamping bolts are tightened after cleaning. Overtightening can cause damage to the plates, as can an incorrect tightening procedure. A torque spanner can be used as directed when clamping bolts are tightened; cooler stack dimensions can also be checked.

In some cases, (installation on board a ship, when processing corrosive liquids, etc.) it may be practical to place the heat exchanger in a DRAINAGE BOX (with capacity for the total volume of the heat exchanger). The outlet of the drainage box should be generously 2” (50mm) dimensioned, not less than (2”)

At the end, we will discuss some common faults in the operation of plate heat exchangers.

Fault Detection