Principle and capacity of heat exchangers
- Capacity of heat exchangers
- Efficiency of plate heat exchangers
- Plate thickness and channel patterns (H, M or L)
- Circulation pumps
- Heat loss of the exchanger
- Connecting the ports
Customers usually request heat exchangers for equipment
- Without a phase change: typically water/water for heating or as oil/water cooler.
- Sometimes the phase changes in the exchanger:
- evaporator (the refrigerant evaporates in the exchanger, i.e. it passes from the liquid to the gaseous phase), or
- condenser (the refrigerant in the exchanger condenses, i.e. it changes from the gas phase to the liquid phase).
Plate heat exchanger capacity
Performance is what most users are most interested in. However the transmitted power is not specified for a given heat exchanger type: one heat exchanger can carry 20 or 100 kW depending on the temperatures and media used. Performance depends on a number of parameters and it is necessary to assess a specific application. The transmitted power depends on
- heat exchanger size (width, height),
- plates count,
- channel patterns,
- volume flow rates,
- temperature gradient between primary and secondary circuit,
- physical properties of the media such as the liquid or gas.
Heat exchanger size
The heat exchanger is usually connected in countercurrent. With such a connection, it is much more efficient than with a cocurrent connection. If the plates are long, then the heat is transferred over a long length and, thanks to the countercurrent connection, primary-inlet and the secondary-outlet temperatures can almost be met. However, a long exchanger also puts up more resistance and a more powerful pump will be needed to overcome the pressure losses. Pressure losses can be reduced by adding the number of plates: doubling the number of plates will reduce the pressure losses to a quarter.
Heat exchanger efficiency
The ideal exchanger has an efficiency of 100%. A real heat exchanger does not achieve such efficiency. However, a plate heat exchanger is the most efficient heat exchanger commonly available. Typically, efficiencies greater than 90% are achieved. In practice, this means that the cold side is heated to a higher temperature than if the two media were mixed in a container. That can be achieved by connecting as a countercurrent heat exchanger.
To achieve high heat exchanger efficiency, it is necessary to provide a large heat exchange area. Inside the heat exchanger, the plates are stacked in many layers: a circuit of hot and cold medium alternates. The plates are made of stainless steel, mostly AISI 316 with a thickness of 0.5 mm.
To increase the heat exchange area, the plates are ribbed and the media flow in the channels formed by these ribs. Such design also brings turbulence in the flow. As a result, the plate heat exchanger has a self-cleaning ability. The plates are arranged in the exchanger so that the two independent flows are separated. The hot or cold medium flows alternately in every other layer and the media are separated by plates and cannot get mixed.
Solder (copper, nickel) is usually used to seal the individual chambers between plates. Solder also participates in heat exchange. For gasketed heat exchangers, a rubber seal is used instead of solder. Gasketed heat exchangers can be opened and cleaned, and it is also possible to additionally increase their capacity by adding additional plates.
When designing the heat exchanger, the optimum between price and efficiency is sought. In many applications, it is not worth aiming towards extreme efficiency, as unused heat is not waste, but returns to the source. Each additional increase in heat exchanger efficiency is more and more expensive, as the graph illustrates:
Heat exchanger plates count
By increasing the plates count, the performance can undoubtedly be increased. Initially, the performance increases, further addition of plates increases the performance more slowly. In order to increase the performance with increasing number of plates, it is necessary to increase the flow rates. If the number of plates and the flow rates double, then the transmitted capacity also doubles.
The longer the heat exchanger, the greater the efficiency (approaching the temperatures of primary-secondary circuits). However, more efficient heat exchangers have higher pressure losses. Pressure losses can be reduced by increasing the number of plates.
Another improvement in efficiency can be achieved by placing two exchangers in a row (in parallel). This approach only makes sense for small flow rates, as pressure losses increase. It is possible to make an exchanger, which encapsulates both circuits, thus having 4 connections in total.
Plate thickness
The next factor determining the efficiency of the exchanger is the thermal conductivity. Smaller board thickness increases efficiency but reduces mechanical and chemical resistance.
Channel patterns
Most manufacturers supply heat exchangers with L, M or H herringbone patterns, or in a combination of L + M or M + H. These channel types can be explained as
- H-plates type (high theta, large angle): the arrangement of the channels between the plates poses an obstacle to the medium, this causes a turbulent flow. As a result the unit is characterized by high efficiency and also by high resistance (i.e. large pressure losses);
- M-plates type (medium, medium angle);
- L-plates type (low theta, sharp/small angle): small resistance and also small efficiency.
Turbulent flow
The suitability of the channels is based on the calculation for the given project. Large pressure losses mean more work for the circulation pump. If the pressure losses are too high, they can be reduced by increasing the number of plates. Excessive reduction of pressure losses is not desirable: in such a case, the small flow between the plates suppresses the turbulent flow and reduces the shear stress on the plates. Turbulent flow prevents dirt from depositing in the heat exchanger. The Reynolds number and shear stress are listed in the calculation report. Turbulent flow occurs if R > 150. The shear stress should be designed at 35, ideally at 50 Pa. If there is a risk of fouling, the exchanger should not be oversized. Our calculation software will help you find which channels are most suitable for your project.
Flow rate
By regulating the flow, the output of the exchanger can be increased or decreased. If the flow on both circuits is doubled, the transmitted power can be doubled. However, it cannot be increased beyond all limits. This is because pressure losses (heat exchanger resistance) increase with the square of the flow and it would be expensive to purchase and operate such a circulation pump. An investment in a circulating pump could exceed the price of the exchanger. In normal cases, it is therefore appropriate to design for flow rates of 1 to 3 m3/h and pressure losses of 20 to 40 kPa.
Heat losses of plate exchanger
The SWEP plate heat exchanger has two circuits:
- internal (has connections on the left) is usually primary: it has no end plates, all plates are inner;
- the outer one (with connections on the right) is usually secondary: the first and last plates serve at the same time as the end plates of the exchanger.
Greater efficiency is usually achieved when hot medium is connected to the internal circuit. Then not so much heat is released into the surrounding space. For the same reason, it is also good to insulate the heat exchanger (we sell dedicated insulation). Also, insulation prevents condensation of water from the air on the unit.
Influence of temperatures on heat exchanger efficiency
It is easier to achieve high efficiency if there is a large temperature difference between the hot and cold circuit. Even a small heat exchanger will transmit a large output.
It is easier to transfer heat when heating cold water from a well than when heating the return of a central heater (provided that the heat source is the same in both cases). Example:
- source (primary circuit) power of 23 kW, 70/50 °C:
- a) secondary circuit 10/40 °C: heat exchanger E5Tx10 (10 plates);
- b) secondary circuit 40/60 °C: heat exchanger E5Tx70 (70 plates).
In both cases, the flow rates are 1 m3/h and the output of 23 kW is transmitted. However, if the cold side is far from warm, the task is simpler and a much smaller exchanger is sufficient.
Conversely, if the temperatures are similar, the efficiency is low and a larger exchanger is needed. In some cases, efficiency can be increased by using a two-way heat exchanger: this is a heat exchanger in which two circuits are in fact connected in series.
Calculation
There are even more parameters that affect the transmitted power. The most important ones are summarized above. We recommend that you request a calculation from us for your project. The following links will help you find your way:
- online calculator of exchanger water/water (can also be used for cooler),
- online calculator of plate cooler oil/water,
- table of capacities for BPHE SWEP (90/70 °C hot, 60/80 °C cold side),
- table of capacities for gasketed exchangers ARES (90/80 °C hot and 75/85 °C cold side).
Connecting plate exchanger
- Never expose the plate exchanger to excessive pulsations (i.e. cyclic pressure or temperature changes).
- The media must not freeze.
- If there is a risk of vibrations transferred to the exchanger, install vibration absorbers.
- The inlets must be equipped with control valves (slowly open the inlets at start-up while the outlet is fully open).
- Install the drain valve on the underside of the lower connection and the vent valve on the upper side of the connection (at the highest point). Open the vent valves during start-up.
- If it might, even occasionally, happen that the pressure in the system is higher than the design pressure of the exchanger, then equip the inlets with a pressure relief valve.
- Internal circuit: has connections on the left hand side when the heat exchanger is in vertical position. The inner circuit is usually one plate smaller and does not adjoin the outer environment. Usually a heat source or refrigerant is connected to it.
- Outer circuit: has connections on the right hand side when the heat exchanger is in vertical position.
- The heat exchanger is in the vertical position if the arrow(s) on the heat exchanger point upwards.
- Heater: positioning and connection have little effect on operation. The exchanger can be placed vertically or horizontally, the inlet can be selected at the top and bottom. It is recommended to place the exchanger so that the hot circuit enters F1, exits F3; the cold circuit enters F4, exits F2. Any limescale deposits have difficult access to the heat exchanger. The outer circuit uses end plates, so it should be used for cold media (otherwise heat would escape into the room). Asymmetric heat exchanger (e.g. E8ASH, E8LASH): the inner circuit (F1-F3) is narrower and has higher pressure losses. In the case of a flow heater, the heated DHW is therefore connected to the outer, i.e. wider circuit (F4-F2).
- Cooler: positioning and connection have little effect on operation. The exchanger can be placed vertically or horizontally, the inlet can be selected at the top and bottom. The outer circuit uses end plates, so it should be used for the hot medium (heat will also escape into the surrounding space).
- Condenser: refrigerant must enter at the top and condensate is drained at the bottom. If terminals F1 and F3 are the same, the exchanger can be rotated 180 degrees. For the steam/water exchanger, see the notes in our steam exchanger article.
- Evaporator: the liquid refrigerant must enter at the bottom, evaporate and the vapors are discharged at the top. Some evaporators have an inlet F3 equipped with a special distribution device for even distribution of the refrigerant. To ensure proper operation, design the system so that the refrigerant speed is 10 to 25 m/s at the inlet and 5 to 10 m/s at the outlet (2.5 to 5 m/s if the connection is horizontal); this also prevents the accumulation of refrigerant oil in the heat exchanger.
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