The theory behind heat transfer

Plate heat exchangers Inside view Heat transfer theory

2 Heat transfer theory The natural laws of physics always Heat transfer theory Heat exchangers allow the driving energy in a system to Heat can be transferred by three Heat transfer theory flow until equilibrium is reached. Heat ­methods. Heat exchanger types leaves the warmer body or the hottest fluid, as long as there is a temperature • Radiation – Energy is transferred by 4 Calculation method difference, and will be transferred to the electromag­netic radiation. One example Temperature program cold medium. is the heating of the earth by the sun. Heat load Logarithmic mean temperature A heat exchanger follows this principle • Conduction – Energy is transferred difference in its endeavour to reach equalisation. between solids or stationary fluids by Thermal length With a plate type heat exchanger, the the movement of atoms or molecules. Density heat penetrates the surface, which Cooling separates the hot medium from the • Convection – Energy is trans- Flow rate cold one very easily. It is therefore ferred by mixing part of a medium with Pressure drop possible to heat or cool fluids or gases another part. Specific heat which have minimal energy levels. Viscosity The theory of heat transfer from one a) Natural convection, where the move- Overall heat transfer coefficient media to another, or from one fluid to ment of the media depends entirely Calculation method another, is determined by several basic upon density difference, and tempera- Construction materials rules. ture differences are evened out. Pressure and temperature limitations • Heat will always be transferred from b) Forced convection, where the move- Fouling and fouling factors a hot medium to a cold medium. ment of the media depends entirely or partly upon the results of an outside 8 Product range • There must always be a temperature influence. One example of this is a difference between the media. pump causing movement in a fluid. 9 Applications Heat exchanger selection • The heat lost by the hot medium is water/water equal to the amount of heat gained by Heat exchanger selection the cold medium, except for losses to water/oil the surroundings. Heat exchanger selection water/glycol Heat exchangers A heat exchanger is a piece of equip- 10 Plate heat exchanger ment that continually transfers heat construction from one medium to another. Plate heat exchanger components Brazed plate heat exchangers There are two main types of heat Fusion-bonded plate heat exchangers. exchangers • Direct heat exchanger, where both 11 Assembly media are in direct contact with each other. It is taken for granted that the 11 Installation media are not mixed together.

An example of this type of heat exchanger is a cooling tower, where water is cooled through direct contact with air.

• Indirect heat exchanger, where both media are separated by a wall through which heat is transferred.

Radiation

2 Alfa Laval heat exchangers Heat exchanger types • Thin material for the heat transfer • Flexibility − the plate heat exchanger In this brochure only indirect heat surface − this gives optimum heat consists of a framework containing exchangers are discussed, i.e. those transfer, since the heat only has to several heat transfer plates. It can ­easily where the media are not mixed, but ­penetrate thin material. be extended to increase capacity. where the heat is transferred through Furthermore, it is easy to open for the heat transfer surfaces. • High turbulence in the medium − purpose of cleaning. (This only applies this gives a higher convection, which to gasketed heat exchangers, and not Temperature losses through radiation results in efficient heat transfer between to brazed or fusion bonded units.) can be disregarded when considering the media. The consequence of this heat exchangers in this brochure. higher heat transfer coefficient per unit • Variable thermal length − most of the Indirect heat exchangers are available area is not only a smaller surface area plate heat exchangers manufactured by in several main types (plate, shell-and- requirement but also a more efficient Alfa Laval are available with two different tube, spiral etc.) In most cases the operation. pressing patterns. When the plate has plate type is the most efficient heat a narrow pattern, the pressure drop is exchanger. Generally it offers the best The high turbulence also gives a higher and the heat exchanger is more solution to thermal problems, giving the self-cleaning effect. Therefore, when effective. This type of heat exchanger widest pressure and temperature limits compared to the traditional shell-and- has a long thermal channel. within the constraint of current equip- tube heat exchanger, the fouling of the ment. The most notable advantages of heat transfer surfaces is considerably When the plate has a wide pattern, a plate heat exchanger are: reduced. This means that the plate heat the pressure drop is smaller and the exchanger can remain in service far heat transfer coefficient is accordingly • Takes up much less space than longer between cleaning intervals. somewhat smaller. This type of heat a traditional shell-and-tube heat exchanger has a short thermal channel. exchanger. When two plates of different pressing patterns are placed next to each other, the result is a compromise between long and short channels as well as between pressure drop and effective- ness.

Convection Conduction

Alfa Laval heat exchangers 3 Calculation method

To solve a thermal problem, we must Temperature program Heat load know several parameters. Further data This means the inlet and outlet Disregarding heat losses to the can then be determined. The six most ­temperatures of both media in the heat ­atmosphere, which are negligible, the important parameters are as follows: exchanger. heat lost (heat load) by one side of a plate heat exchanger is equal to the • The amount of heat to be transferred T1 = Inlet temperature – hot side heat gained by the other. The heat load (heat load). T2 = Outlet temperature – hot side (P) is expressed in kW or kcal/h. T3 = Inlet temperature – cold side • The inlet and outlet temperatures on T4 = Outlet temperature – cold side Logarithmic mean the primary and secondary sides. temperature difference Logarithmic mean temperature difference • The maximum allowable pressure drop The temperature program is shown in (LMTD) is the effective driving force in the on the primary and secondary sides. the diagram below. heat exchanger. See diagram to the left. • The maximum operating temperature. Temperature Temperature Thermal length • The maximum operating pressure. Thermal length (Θ) is the relationship between temperature difference δt on • The flowrate on the primary and T1 one side and LMTD. ­secondary sides. ∆T1 δt Θ = LMTD If the flow rate, specific heat and T4 T2 ­temperature difference on one side are Thermal length describes how difficult a known, the heat load can be calculated. ∆T2 duty is from a thermal perspective. See also page 6. T1 - T2 LMTD = ∆ ∆ T3 In ∆T1 Density ∆T2 Density (ρ) is the mass per unit volume and is expressed in kg/m3 or kg/dm3.

4 Alfa Laval heat exchangers Cooling Flow rate Specific heat

For some duties, cooling applications This can be expressed in two different Specific heat (cp) is the amount of for example, the temperature program terms, either by weight or by volume. energy required to raise 1 kg of a sub- is very tight with close approaches on The units of flow by weight are in kg/s stance by one degree centigrade. The the different temperatures. This gives or kg/h, the units of flow by volume specific heat of water at 20°C is 4.182 what we refer to as high theta duties in m3/h or l/min. To convert units of kJ/kg °C or 1.0 kcal/kg °C. and requires high theta units. High theta volume into units of weight, it is neces- duties are duties that have Θ > 1 and sary to multiply the volume flow by the Viscosity are characterized by: density. Viscosity is a measure of the ease of flow of a liquid. The lower the viscosity, • Long plate, longer time for the fluid The maximum flow rate usually deter- the more easily it flows. to be cooled mines which type of heat exchanger is the appropriate one for a specific pur- Viscosity is expressed in centiPoise (cP) • Low pressing depth that gives less pose. Alfa Laval plate heat exchangers or centiStoke (cSt). fluid per plate to be cooled can be used for flow rates from 0.05 kg/s to 1,400 kg/s. In terms of volume, Overall heat transfer coefficient Plate heat exchangers are superior this equates to 0.18 m3/h to 5,000 Overall heat transfer coefficient (k) is a compared to shell-and-tube heat m3/h in a water application. If the flow measure of the resistance to heat flow, exchangers when it comes to theta rate is in excess of this, please consult made up of the resistances caused values. Shell-and-tube heat exchangers your local Alfa Laval representative. by the plate material, amount of foul- can go up to a maximum value of theta ing, nature of the fluids and type of ~1 while plate heat exchangers goes up Pressure drop exchanger used. to theta values of 10 and more. For a Pressure drop (∆p) is in direct rela- shell-and-tube to climb over theta value tionship to the size of the plate heat Overall heat transfer coefficient is of 1 or more, several shell-and-tube exchanger. If it is possible to increase expressed as W/m2 °C or kcal/h, m2 °C. needs to be put in series. the allowable pressure drop, and inci- dentally accept higher pumping costs, Temperature Temperature then the heat exchanger will be smaller and less expensive. As a guide, allow- able pressure drops between 20 and T1 in Lower Θ 100 kPa are accepted as normal for water/water duties. ∆1

T2 out T1 out

∆2

T2 in

The diagram shows that large temperature differences give low theta.

Temperature Temperature

T1 in Higher Θ ∆1 T2 out T1 out ∆2 T2 in

The diagram shows that small temperature differences give large theta.

Alfa Laval heat exchangers 5 Calculation method The heat load of a heat exchanger can be derived from the following two formulas:

1. Heat load, Theta and LMTD calculation P P P = m · cp · δt (m = ; δt = ) cp · δt m · cp P = k · A · LMTD

Where: P = heat load (kW) m = mass flow rate (kg/s)

cp = specific heat (kJ/kg °C) δt = temperature difference between inlet and outlet on one side (°C) k = heat transfer coefficient (W/m2 °C) A = heat transfer area (m2) LMTD = log mean temperature difference

δt k · A Θ = Theta-value = = LMTD m · cp T1 = Temperature inlet – hot side T2 = Temperature outlet – hot side T3 = Temperature inlet – cold side T4 = Temperature outlet – cold side

LMTD can be calculated by using the following formula, where ∆T1 = T1–T4 and ∆T2 = T2–T3

LMTD = ∆T1 - ∆T2 In ∆T1 ∆T2

2. Heat transfer coefficient and design margin

The total overall heat transfer coefficient k is defined as:

1 1 1 δ 1 Where: — = — + — + — + R = — + R k f f α1 α2 λ kc

The design margin (M) is calculated as: M = kc - k k

2 α1 = The heat transfer coefficient between the warm medium and the heat transfer surface (W/m °C) 2 α2 = The heat transfer coefficient between the heat transfer surface and the cold medium (W/m °C) δ = The thickness of the heat transfer surface (m) 2 Rf = The fouling factor (m °C/W) λ = The thermal conductivity of the material separating the medias (W/m °C) 2 kc = Clean heat transfer coefficient (Rf =0) (W/m °C) k = Design heat transfer coefficient (W/m2 °C) M = Design Margin (%)

Combination of these two formulas gives: M = kc · Rf

i.e the higher kc value, the lower Rf-value to achieve the same design margin.

6 Alfa Laval heat exchangers Every parameter in the equation Construction materials Fouling and fouling factors beside can influence the choice of heat High quality AISI 316 stainless steel Fouling allowance can be expressed exchanger. The choice of materials does plates are used in most Alfa Laval heat either as a design margin (M), i.e. an not normally influence the efficiency, only exchangers for water/water applica- additional percentage of heat transfer the strength and corrosion properties of tions. When the chloride content as area, or as a fouling factor (Rf) expressed the unit. shown in the tables on page 9 does in the units m² °C/W or m²h °C/kcal.

not require AISI 316, the less expensive Rf should be much lower for a plate In a plate heat exchanger, we have stainless steel material AISI 304 may heat exchanger than for a shell-and- the advantages of small temperature sometimes be used. Several other plate tube exchanger. There are two main differences and plate thicknesses of materials are also available for various reasons for this. between 0.3 and 0.6 mm. The alpha applications. For Alfa Laval brazed and values are products of the very high tur- fusion bonded plate heat exchangers 1. Higher k-values means bulence, and the fouling factor is usually AISI 316 is always used. For salt water lower fouling factors. very small. This gives a k-value which and brackish water only titanium should The design of plate heat ­exchangers under favourable circumstances can be be used. gives much higher turbulence, and in the order of 8,000 W/m2 °C. thereby thermal effeciency, than a shell- Pressure and temperature and-tube exchanger. A typical k-value With traditional shell-and-tube heat limitations (water/water) for a plate heat exchanger exchangers, the k-value will be below The maximum allowed temperature and is 6,000-7,500 W/m² °C while a 2,500 W/m2 °C. pressure influence the cost of the heat typical shell-and-tube exchanger gives exchanger. As a general rule, the lower only 2,000-2,500 W/m² °C. A ­typical Important factors to minimize the heat the maximum temperature and maxi- Rf -value used for shell-and-tube exchanger cost: mum pressure are, the lower the cost exchangers is 1 x 10-4 m² °C/W. With of the heat exchanger will be. k-values 2,000-2,500 W/m² °C this give 1. Pressure drop a Margin of 20-25%. (M = kc x Rf). To The larger allowed pressure drop the achieve M = 20-25% in the plate heat smaller heat exchanger. exchanger with 6,000-7,500 W/m² °C -4 the Rf -value should only be 0.33 x 10 2. LMTD m² °C/W. The larger the temperature difference between the medias is, the smaller the 2. Difference in how margin is added. heat exchanger will be. In a shell-and-tube heat exchanger margin is often added by increasing the tube length, keeping the same flow through each tube. In a plate heat exchanger however, margin is added by adding parallell channels, i.e. lowering the flow per channel resulting in lower turbulence/efficiency, increasing the risk for fouling. A too high fouling factor can result in increased fouling!

For a plate heat exchanger in a water/ water duty a margin of 0-15% depending on water quality is normally enough.

Alfa Laval heat exchangers 7 Product range

The plate heat exchangers in this ­brochure are suitable for the majority of relatively uncomplicated heat transfer jobs using water, oil or glycol as the media. When it comes to the effective- ness of heat transfer and economical operation, the plate heat exchanger is unsurpassed in HVAC, refrigeration, sanitary water heating as well as indus- trial heating and cooling applications.

The Alfa Laval product range of plate heat exchangers is extensive. From the largest units with maximum surfaces up to 3,000 m² and flow rates of approxi- mately 5,000 m³/h to smaller units with maximum heat transfer areas of less than 1 m² and flow rates from 0.18 m³/h.

Every single heat exchanger in the cata- Gasketed plate heat exchangers logue can perform a range of duties. Applications include the heating and cooling of different fluids in factories, HVAC applications, process cooling, components in air conditioning equip- ment etc. The list of applications is ­considerable. Not all types of our plate heat exchangers are included in this bro- chure. If you require more information, please do not hesitate to contact us.

Brazed plate heat exchangers

Fusion-bonded plate heat exchangers

8 Alfa Laval heat exchangers Applications

Although the principle of heat transfer is the same irrespective of the medium used, we must differentiate the applications from each other. Most duties fall into three main applications:

Water/Water Plate material Maximum temperature °C The largest part of our production of heat Some typical uses of Chloride plate heat exchangers exchangers is used for water/water duties, content (ppm) 60ºC 80ºC 100ºC 120ºC i.e. water heated or cooled with water. This • District heating/cooling can be achieved by different methods: • Tap water heating 10 ppm 304 304 304 316 • Swimming pool heating 25 ppm 304 304 316 316 Water must be cooled • Heat recovery (engine cooling) 50 ppm 316 316 316 Ti Here, water with a lower temperature is • Temperature control of fish farms 80 ppm 316 316 316 Ti • Steel industry – furnace cooling used, for exam­ple from a cooling tower, 150 ppm 316 Ti Ti Ti • Power industry – central cooling lake, river or sea. 300 ppm Ti Ti Ti Ti • Chemical industry – process cooling Water must be heated Here, water with a higher temperature is Gasket Nitrile used, for example district heating, boiler or material EPDM hot process water.

Water/Oil Plate material Maximum temperature °C In some industries, oil has to be cooled Some typical uses of using water. This water can then be plate heat exchangers Chloride ­connected to a heat recovery ­system • Hydraulic oil cooling content (ppm) 60ºC 80ºC 100ºC 120ºC recovering the heat from the oil to ­various • Quench oil cooling 10 ppm 304 304 304 316 • Cooling of motor oil in engine test beds usages, such as tap water heating etc. 25 ppm 304 304 316 316 With synthetic oil it may be necessary 50 ppm 316 316 316 Ti to use special gaskets. Please contact 80 ppm 316 316 316 Ti Alfa Laval for these applications. 150 ppm 316 Ti Ti Ti 300 ppm Ti Ti Ti Ti Plate heat exchangers can function with oils having viscosities as high as 2,500 Gasket Nitrile centiPoise. Emulsions can also be used in material plate heat exchangers, and can be treated like water when concentrations are below 5%.

Water/Glycol Plate material Maximum temperature °C When there is a risk of freezing, add glycol Some typical uses of Chloride plate heat exchangers to the water. content (ppm) 60ºC 80ºC 100ºC 120ºC • As an intercooler in a heat pump Glycol has a different heat ­capacity from • Chilled water production in food 10 ppm 304 304 304 316 water and therefore needs a somewhat factories 25 ppm 304 304 316 316 larger heat transfer area to perform the • Cooling of air conditioning circuits 50 ppm 316 316 316 Ti same duty. On the other hand, the physical • Solar heating systems 80 ppm 316 316 316 Ti properties of the various glycols are much 150 ppm 316 Ti Ti Ti the same. Examples of glycols are: 300 ppm Ti Ti Ti Ti

• Ethylene glycol (mono, di or tri) Gasket Nitrile • Propylene glycol. material EPDM

Alfa Laval heat exchangers 9 Plate heat exchanger construction

A plate heat exchanger consists of a number of heat ­transfer plates which are held in place between a fixed plate and a loose pressure plate to form a complete unit. Each heat transfer plate has a gasket arrangement which provides two ­separate channel systems.

The arrangement of the gaskets (field and ring gaskets) results in through flow in single channels, so that the primary and ­secondary media are in counter-current flow. The media cannot be mixed because of the gasket design.

The plates are corrugated, which ­creates turbulence in the fluids as they flow through the unit. This turbulence, in association with the ratio of the ­volume of the media to the size of heat exchanger, gives an effective heat transfer coefficient.

Plate heat exchanger components The components consist of a fixed end plate, connections and a loose pres- sure plate, with carrier bars mounted between them. The plates are hung from the top carrier bar. The carrier bars also serve to position the heat transfer plates. The single plates are pulled together to form a plate pack by means of tightening bolts.

Gasketed plate heat exchangers are available in standard sizes or can be individually prepared.

Gaskets

Materials available Nitrile rubber general purpose, oil resistant EPDM general purpose, elevated temperatures HeatSealF™ for high temperatures, specially heating by steam

Brazed plate heat exchangers Fusion-bonded plate heat exchanger A brazed plate heat exchanger is small, AlfaNova is a new type of plate heat light and compact. It does not need exchanger constructed of 100% stain- gaskets. Instead, it is brazed together less steel using AlfaFusion. A unique using copper to give a strong, compact bonding technology that provides high construction. temperature resistance (up to 550ºC) and an exceptional level of hygiene. Copper- This heat exchanger is especially suit- free, AlfaNova offers unmatched corro- able for pressures up to 50 bar and sion resistance. temperatures from -196°C to +550°C.

10 Alfa Laval heat exchangers Assembly

Alfa Laval delivers your heat exchanger assembled and pressure tested. Gasketed heat exchangers can easily be opened for inspection and ­cleaning. Should the capacity requirements change in the future, additional plates can easily be hung in the frame on site.

The following sketches show assembly step by step:

1. The frame is put together. It consists of 2. Then the plates ­corresponding to the 3. The tightening bolts are fitted and frame and pressure plates, top and bot- platage specification are positioned the plate pack is tightened by means tom carrying bars and connections. The in the frame. of a spanner or any other suitable end plate is the first plate to be hung in tool to a set measure (specified in the frame. the platage specification).

Installation

All the heat exchangers in this brochure The inlet of one medium is next to have the connections in the frame plate. the outlet of the other. If S1 is the inlet They are referred to as S1, S2, S3 and for medium 1, then S4 is the outlet S4. for medium 2. Every heat exchanger ­delivered is accompanied by instructions The gasketed heat exchanger can as to which inlet and outlet to use. be placed directly on the floor. When ­possible, it is always safer to secure the Depending upon the type of ­connection unit with foundation bolts. The plate heat selected, prepare the pipework with exchanger is noted for occupying less screwed thread ends, fit flanges or space than traditional heat ­exchangers. ­prepare for welding. When planning the space recommended, it is necessary to leave space on one Some of the accessories available for side of the heat exchanger only. The the Alfa Laval plate heat exchangers pipe connections can be either screwed are insulation, drip trays and protection or flanged, depending on the type of sheets. heat exchanger selected.

The brazed plate heat exchanger will normally be built into the pipework, or mounted into a small console. Gasketed plate heat exchanger standing directly on the floor.

Alfa Laval heat exchangers 11 Alfa Laval in brief

Alfa Laval is a leading global provider of specialized products and engi- neered solutions. Our equipment, systems and serv- ices are dedicated to helping custom- ers to optimize the performance of their processes. Time and time again. We help our customers to heat, cool, separate and transport prod- ucts such as oil, water, chemicals, beverages, foodstuffs, starch and pharmaceuticals. Our worldwide organization works closely with customers in almost 100 countries to help them stay ahead.

How to contact Alfa Laval Up-to-date Alfa Laval contact details for all countries are always available on our website at www.alfalaval.com

Alfa Laval reserves the right to change specifications without prior notification.

© 2004 Alfa Laval

ECF00250EN 1009 ALFA LAVAL is a trademark registered and owned by Alfa Laval Corporate AB