The laws of physics explain that the energy of a system continues to flow until it is in equilibrium. According to the law of thermodynamics, the heat flows from one system to another due to heat or temperature differences between both systems. The heat exchanger works according to the equilibrium principle. The heat exchangers play the most important roles in heat transfer from one fluid to other. The heat exchangers have different types, and a plate heat exchanger (PHE) is one of them. The plate heat exchanger extracts heat from the surface and separates the cold fluid from the hot fluid. This article mainly explains various aspects of the plate heat exchanger.
What is a Plate Heat Exchanger?
A plate heat exchanger uses a series of metal plates to move heat from one fluid to other. These plates are positioned over each other to create a channel’s series so that fluid can move between them. In the 1920s, Dr. Richard Seligman invented the plate heat exchanger (PHE).
The main benefit of the plate heat exchangers over conventional heat exchangers is that the fluid is distributed over the plate due to that fluid is exposed to a larger surface area. This increases the heat transfer rate and significantly speeds up the rate of temperature change.
Nowadays, plate heat exchangers are widely used, and very small welded designs utilize millions of combi boilers in the hot water section. The high heat transfer efficiency with such a small size increases the flow of domestic hot water (DHW) in a combi boiler.
Small plate heat exchangers have a big influence on household heating and hot water. The large business models utilize gaskets between the plates, while the smaller models tend to weld.
Working of Plate Heat Exchanger
A plate heat exchanger is a type of heat exchanger that contains a series of metal plates to transmit heat between fluids. The plate heat exchanger works on the principle of thermodynamics. In these exchangers, each plate has a confined, hollow tubular shell.
The plates are arranged in such a way that thin rectangular channels are developed to exchange heat through half pieces. The operating fluid moves between these twisted and narrow channels.
The plates of this exchanger are surrounded by gaskets to control the fluid flow. These gaskets arrange in such a way that only one type of liquid (such as oil which is being heated) distributes on one plate and another fluid (such as hot water) distributes on the next plate. The following figure represents two adjacent boards.
After this arrangement, the cold and hot fluids alternately pass through the plate, whereby a heat exchange takes place. The plates have a large surface area; therefore, they offer an excellent heat transfer rate than tubular heat exchangers.
As you can see in the above diagram, the cooling fluid inlet (blue) is on the bottom, the cooling fluid outlet on the top, and contrariwise for the hot fluid outlet (red). The cool fluid flows upward and the fluid to be cooled flows downward, transporting the heat through the plates. After the completion of this process, the heating medium is finally cooled, and the cooling medium is heated.
The heat transfer principle and design of the plate heat exchangers characterize by their compact design, low heat loss, a wide range of applications, flexible operation, high heat transfer efficiency, small installation area, and convenient installation and cleaning functions.
For a better understanding, watch the following video:
Read Also: Working and Types of Shell & Tube Heat Exchanger
Parts and function of the Plate Heat Exchanger
The plate heat exchanger has the following major parts:
- Carrying beam
- Fixed Plate
- Support column
- Pressure Plate
- Guiding Bar
- Tightening Unit
- PHE Plates and gasket
A single plate heat exchanger can contain a maximum of 700 plates. As the plate pack compresses, the holes in the plate’s corners create a constant tunnel or manifold that allows fluid to flow through the plate pack and exit the device.
The space between the thin plates of the heat exchanger makes a tight channel that alternately traverses by cold and hot fluids and offers very small resistance to heat transfer.
2) Carrying Beam
The upper part installs between the supporting column and the fixed plate on which the pressure plates and the exchanger plates are connected.
Read Also: Working of Double Pipe Heat Exchanger
3) Fixed Plate
The fixed plate is a fundamental part of the plate heat exchanger. As the name of this plate represents that it is a fixed frame plate. Generally, the heat exchanger pipes connect with the fixed plates.
4) Support Column
This is a non-moveable part of the PHE. The guiding bar and carrying beam are attached to this part.
Read Also: Types of Shell & Tube Exchanger
5) Pressure Plate
The PHE has a mobile pressure plate frame attached with the exchanger carrying beam. This frame compresses the plates of the exchanger.
6) Guiding Bar
This part guides the pressure plate and heat exchanger plates downward.
7) Tightening Unit
It is used to compress the frame components of the plate pack. It has tightening washers, tightening nuts, and tightening bolts.
8) PHE Plates and Gasket
The packing of plates installs between the pressure plate and the fixed frame plate. This plate pack compresses by tightening the screws fastened between the two plates. The gaskets cover the plates to regulate the flow.
Read More: Working of Gasket
Types of Plate Heat Exchangers
The plate heat exchanger has the following major types:
- Gasketed plate heat exchangers
- Brazed heat exchanger
- Welded plate heat exchangers
- Semi-Welded heat exchanger
- Plate and Frame heat exchanger
1) Gasketed plate heat exchanger
This heat exchanger uses top-quality gaskets and construction. This gasket seals the plates and stops leakage. You can easily remove the plates of this exchanger for the replacement, expansion, or cleaning of the plates, which significantly reduces maintenance costs.
2) Brazed Plate Heat Exchanger
The brazed plate heat exchanger is used in multiple refrigeration and industrial applications. Since the stainless-steel plate is brazed with copper because it has excellent corrosion resistance.
These types of plate heat exchangers are an economically superior option because of their compact design and excellent efficiency.
Advantages of brazed plate heat exchangers:
- It is most commonly used exchanger
- It has low heat loss
- These exchangers have compact design
- They have low costs
3) Welded plate heat exchangers
The working of these heat exchangers is the same as gasket heat exchangers, but the plates of these heat exchangers are welded with each other.
These have excellent durability and are best suitable for transporting hot fluids and corrosive substances. These exchangers have welded plates; therefore, you can’t clean the plate mechanically like the plate and frame heat exchanger.
4) Semi-Welded plate heat exchanger
This heat exchanger is a combination of gasketed plates and welded plates. It has a pair of two plates welded with each other and then a gasket with another pair of plates so that one fluid can flow through welded and the other fluid can flow through the gasketed part.
This arrangement of the plate heat exchanger makes it easy to repair. Therefore, this exchanger can also transfer more powerful fluid on the other.
These exchangers have very little risk of fluid loss and are well suited for the transportation of expensive materials.
5) Plate and Frame Heat Exchanger
The heat exchanger in which the plates create a frame is known as a plate and frame heat exchanger. This exchanger consists of corrugated plates in the frame. This construction produces high wall shear stress and turbulence, leading to high stain resistance and a high heat transfer rate.
This heat exchanger has gaskets. In addition to the sealing effect, the gasket also guides the liquid flow and is installed along the groove on the plate edge.
The plate and frame heat exchanger uses to exchange heat between liquid and liquid at medium to low pressure. You can safely use a plate and frame heat exchanger without a gasket at high temperature and pressure. This type of heat exchanger has high flexibility as plates can be compressed or added in various situations.
The characteristics of the plate and frame heat exchanger are given below:
- The plate and frame heat exchanger has an easy and quick assembly and disassembly.
- It has the capacity to work with different working conditions by removing or adding heat plates to vary the flow rate.
- Gaskets of this exchanger have high costs.
- This heat exchanger limits the maximum temperature and pressure due to the operation of the gasket.
- It has high costs because of its moulds and complex design.
- We cannot use materials that are not suitable for welding, such as titanium.
Design of plate and frame heat exchanger
Plate heat exchangers (PHE) are specifically designed to transfer heat between low-pressure fluid and medium-pressure fluid. Brazed, semi-welded, and welded exchangers use to heat exchange between high-pressure fluids.
Instead of a tube running by the chamber, this exchanger has two alternating chambers, which are usually very thin, the larger surface being separated via a corrugated metal plate.
The plates are made of stainless steel because steel has high corrosion resistance, strength, and temperature resistance.
Plate heat exchangers have multiple plates installed on each other to form a series of channels through which fluid can flow. A rubber gasket uses to separate these plates. This gasket attaches to the parts around the edge of the plate.
The gap between two adjacent plates generates a channel for the flow of the fluid.
The outlet and inlet holes in the plate corners permit the cold and hot medium to pass by the heat exchanger alternating channels so that the plate can always make contact with the cold medium on one end and the hot medium on the other.
The plate heat exchanger uses multiple plates to achieve an exchange area of up to thousands of square meters.
Plate Heat exchanger Heat Transfer
The heat transfer coefficient of the fluid flowing by the plate heat exchanger can be calculated by the below-given formula:
In the above-given equation:
∆Tm = Effective mean temperature difference
A = total area of the plate
U = overall heat transfer coefficient
You may calculate the total area of the plate by the below-given formula:
In the above equation:
Np = number of plates
Ap = each plate area
The coefficient of the overall heat transfer may find by the below-given equation:
hhot = hot fluid’s convective heat transfer coefficient
hcold = cold fluid’s convective heat transfer coefficient
tp = plate thickness
kp = plate conductivity
Rf, hot = hot fluid fouling factor
Rf, cold = cold fluid fouling factor
The heat transfer rate of the heat exchanger can be calculated by the two different approaches:
- log-mean temperature difference (LMTD)
- thermal effectiveness
The below-given formula can calculate the heat transfer by applying the first approach:
ΔTlm = log mean temperature difference
F = correction factor
ΔTlm can calculate by the below given formula:
The above formula represents to temperature difference for the parallel flow heat exchangers. This temperature is further evaluated in the following equation:
The second method to find the heat transfer rate of the plate and frame heat exchanger is the ratio between the actual heat transfer to the highest theoretical heat transfer:
Advantages and Disadvantages of plate heat exchangers
Advantages of plate heat exchanger
- The plate heat exchanger has an easy design.
- These types of heat exchangers have a large heat transfer rate than the shell & tube heat exchangers.
- There is no need for extra space for the exchanger disassembly.
- They have simple maintenance and cleaning.
- The plate heat exchangers have small size than the shell & tube heat exchangers.
- It has a small fouling factor.
- It has easy repairing and washing.
- These exchangers have low installation costs.
Disadvantages of plate heat exchangers
- These exchangers have poor sealing and are easy to leak.
- The plate heat exchangers have large flow resistance than the tube exchangers.
- These have a high-pressure drop.
- It has high clogging index, particularly suspended matters in the fluid.
- The heat resistance of the sealing material limits the operating temperature.
- It has a limited working pressure which is typically less than 1.5 MPa.
- Inadequate sealing can lead to leaks and make replacement difficult.
Applications of Plate Heat Exchanger
The plate heat exchangers are used in the following applications:
- Heat pump isolation
- Mash Coolers
- Glycol Coolers
- Cooling tower isolation
- Lube Oil Coolers
- Batch Heating & Cooling
- Free cooling
- Heat Recovery Interchangers
- Process Heating & Cooling
- Water heaters
- Waste heat recovery
Are plate heat exchangers efficient?
The plate heat exchangers are one of the most efficient heat exchangers. The efficiency rate of the plate heat exchanger is approx—90%.
What are plate heat exchangers used for?
A plate heat exchanger is a well suitable exchanger to exchange heat between low pressure and medium pressure fluids. They are used in boilers, compressors, free cooling, and mash coolers applications.
Which heat exchanger is best?
Plate heat exchangers are most effective because of the turbulence on both sides. High turbulence and high heat transfer rate are important for even flow distribution. Plate heat exchangers are limited to low viscosity fluids.
Who invented the plate heat exchanger?
In 1923, Dr Richard Seligman was invented the plate heat exchanger.
How long does a plate heat exchanger last?
A heat exchanger can continuously work for up to 10 years.