29. Heat Exchangers Characteristics.

Specifications for waste heat recovery unit Commercial heat exchanger equipment

Low temperature from o°C to -120°C

Intermediate temperature 120°C – 650 °C

High temperature above 650°C

Recovers moisture

Large temperature differentials permitted

Packaged units available

Can be retrofit

No cross contamination

Compact size

Gas-to-gas heat exchange

Gas-to-liquid heat exchanger

Liquid-to-liquid heat exchanger

Corrosive gases permitted with special construction

shell-and-tube exchanger

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Finned-tube heat exchanger

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Waste-heat boilers

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Spiral heat exchanger

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Concentric tube heat exchanger

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Concentric tube heat exchanger recuperator

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Plate heat exchanger

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Run around heat exchanger

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Heat wheel metallic

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Heat wheel hygroscopic

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Heat wheel ceramic

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Heat pipe

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* Can be constructed of corrosion-resistant materials, but consider possible.

** Off-the-shelf items available in small capacities only.

*** Controversial subject. Some authorities claim moisture recovery. Do not advise depending on it.

**** With a purge section added. Cross-contamination can be limited to less than 1% by mass.

***** Allowable temperatures and temperature differential limited by the phase equilibrium properties of the internal fluid.

The basic design equations of a heat exchanger in which heat transfer occurs between two fluid streams that are separated by a heat transfer surface are:

• thermal balance equation:

[kW] (13)

• heat exchange equation:

[kW] (14)

where: T_cin, T_cout – temperatures of cold fluid at heat exchanger inlet and outlet, respectively, K;

T_hin, T_hout – temperature of hot fluid at heat exchanger inlet and outlet, respectively, K;

, - mass flow rates of cold and hot fluids, respectively, kg/s;

, - specific heats of cold and hot fluids, respectively, kJ/(kg·K);

- heat flow lost through the walls. It represents about 1.5% from the heat flow released by hot fluid;

U – overall heat transfer coefficient, kW/(m²·K);

A – heat transfer area, m²;

LMTD – logarithmic mean temperature difference:

(15)

=max(T_hin-T_cout, T_hout-T_cin) for parallel flow;

=T_hin-T_cin for counterflow;

=min (T_hin-T_cout, T_hout-T_cin) for parallel flow;

=T_hout-T_cout for counterflow.

Figure 4 illustrates the temperature profiles of the respective hot and cold fluids passing through a heat exchanger.

the overall coefficient of heat exchanger is calculated by equation:

• for exchangers with flat heating surface:

[(m²·K)/kW] (16)

• for tubular exchangers:

[(m²·K)/kW] (17)

where: α_h, α_c – coefficients of heat transfer by convection from the hot fluid to wall and from the wall to cold fluid, respectively, kW/(m²·K);

λ – thermal conductivity of material wall separating the fluids, kW/(m·K);

δ – separating wall thickness, m;

α_i, α_e – coefficients of heat transfer by convection between fluid and inner and outer surface of pipe, respectively, kW/(m²·K);

d_i, d_e – inner and outer diameters of pipe, respectively, m.

Figure 4. Temperature variation through heat exchangers.

The overall heat transfer coefficient of a heat exchanger which presents deposit on heat transfer surface can be calculated with the equation:

[(m²·K)/kW] (18)

where: R_sdi, R_sde – thermal resistances of deposits on inner and outer surface respectively, kW/(m²·K).

When the heat exchanger presents multiple passes and/or cross flow, the LMTD has to be corrected by factor F which varies from unitary to zero depending on the following ratios:

; (19)

where T_t and T_s are the temperatures of tube and shell flows respectively. The values of correction factor F are given in graphs drawn for various flow configurations as function of P and R.

The heat exchanger effectiveness is defined as the ration between the actual heat transferred and maximum heat which could possibly be transferred from one stream to the other:

(20)

where: C_min = min(C_h; C_c); ;

The heat transferred by a heat exchanger can be expressed as follow:

(21)

The heat exchanger effectiveness can be expressed by using the number of transfer units (NTU) in this way:

• for parallel flow heat exchangers:

(22)

• for counter current flow heat exchangers:

(23)

where

Recuperators

Recuperators are used to recover the waste heat at medium to high temperature level. They can use radiation, convection or combination of the heat transfer mechanisms. The simplest configuration for a recuperator consists of two concentric metal tubes (Fig. 5). The hot exhaust gases pass through the inner tube and the cold air from atmosphere passes through the external annulus. In a convective heat exchanger, known as shell and tube exchanger, the hot gases are carried through a number of parallel small diameter tubes, while the incoming air to be heated enters a shell surrounding the tubes and passes over the hot tubes one or more times (depending on the number of baffles) in the direction normal to their axes (Fig. 6).

Figure 5. Metallic radiation recuperator [7].	Figure 6. Convection recuperator [7].

The hybrid recuperators combine the radiation and convective transfer designs to increase the effectiveness of heat transfer. The radiation section placed at higher temperature of hot fluid is followed by convective section (Fig. 7).

Recuperators are built from metallic or ceramic materials. Metallic recuperators can operate up to 1000°C, while the ceramic-tube recuperators can operate up to 1550°C for hot-side temperatures and up to 1000°C for cold-side temperatures.

Figure 7. Hybrid (convective-radiative) recuperator [5].

regenerators

Regenerators are heat exchangers that use thermal mass (bricks, ceramic, metal) in an alternating cycle to recover heat from exhaust to preheat supply air. First, the thermal mass is heated by the hot gas and then, air from atmosphere passes over the mass to extract heat.

The most common regenerators are the heat wheels and rotary regenerators. the heat wheels consist of porous disk fabricated from a material with high heat capacity, which rotates between two parallel ducts: one for hot gas and the other for cold gas to be heated (Fig. 8). Firstly, the heat is accumulated in disk sector placed in the hot stream and then the accumulated heat is transferred to cold stream as the disk rotates. They are used in applications where the heat exchange takes place between large masses of gasses (air) having small temperature differences, typically in space heating, curing, drying ovens and heat-treat furnaces. The rotary regenerator has a cylinder made from a matrix which rotates across the hot exhaust gasses and cold air streams and transfers the heat from gasses to air (Fig. 9). It is used especially to recover the heat from exhaust gasses from boiler to preheat the combustion air.

Figure 8. Heat wheel [7].	Figure 9. rotary regenerator [8].

Heat pipes

The heat pipe heat exchangers are heat transfer devices in which the latent heat of vaporization is utilized to transfer heat over a long distance with a corresponding small temperature difference [9]. They consist of closed tube filled with a proper working fluid and having a capillary wick structure on interior wall.

The heat pipes can be divided into three types: conventional heat pipes (Fig. 10), two phase closed thermosyphons (heat pipes without the wick structure) (Fig. 11) and oscillating heat pipes (Fig. 12).

Figure 10. Conventional heat pipe [9].	Figure 11. Two phase closed thermosyphon [9].	Figure 12. oscillating heat pipe [9].

The heat applied to the external surface of pipe causes evaporation of working fluid near the surface to evaporate. This part of heat pipe is called evaporator. Generated vapours have slightly higher pressure and temperature which causes vapours to flow to the opposite end of pipe, where a slightly lower temperature causes the vapour to condense and release its latent heat of vaporization. This part of heat pipe is called condenser. The condensed fluid returns to the evaporator through the capillary action of the wick in the conventional heat pipes or the gravitational force in the two-phase closed thermosyphons.

pulsating heat pipes are one of the latest developments in heat pipe technology. In contrast to classic heat pipes, where the working fluid inside the heat pipe circulates continuously by capillary forces between the heat source and the heat sink in the form of a counter current flow, the working fluid in a pulsating heat pipe oscillates in its axial direction. The basic heat transfer mechanism in a pulsating heat pipe is the oscillating movement of the fluid associated with the phase change (evaporation and condensation) phenomena [9].

The main advantages of heat pipes are:

• high heat recovery effectiveness;

• high compactness;

• lack of moving parts;

• light weight;

• reliability.

The heat pipe exchangers are used in many industries (energy engineering, chemical engineering and metallurgical engineering).

Finned tube heat exchangers

These heat exchangers are formed from coils having fins attached to the external tube surface to provide additional heat transfer area (Fig. 13). They are used for boiler feed water preheating (economisers), for heating the water for space heating, or domestic hot water. The liquid flowing through the tubes receives the heat from the flue gas flowing across the tubes. For every 22°C reduction in flue gas temperature by passing through an economiser or for every 6°C rise in feed water temperature there is 1% saving of fuel in the boiler.

Figure 13. Finned tube heat exchangers.

Plate heat exchangers

A plate heat exchanger consists of a series of thin corrugated parallel sheets forming flow channels (Fig. 14). The waste heat stream (hot fluid) flows downwards through every channel while the cold liquid flows upwards through the intercalated channels forming the counter current arrangement. These heat exchangers are used when the temperature differences are low and large heat exchange surface is required, in liquid/liquid or steam/liquid applications. They can be classified as gasketed, brazed and welded plate heat exchangers. The heat recovery efficiencies range between 75% and 80%. They are simple and easy to clean. The maximum allowed temperature and pressure inside the gasketed heat exchangers is 250°C and 25 bar respectively.

Because the channels are quite narrow, strong vortices are formed leading to high heat transfer coefficients, high pressure drops and reduced fouling

Figure 14. Gasketed plate heat exchanger.

Spiral heat exchangers

A spiral heat exchanger is built from a pair of long flat metal strips that are concentrically rolled to form two channels in a counter-flow arrangement (Fig. 15). Each of outer arms of the spiral has a fluid port tangentially connected and each of inner arms of the spiral has an axial fluid port. The main advantages of these heat exchangers are the compactness, low fouling characteristics, low pressure drop and high thermal efficiency. The heat recovery efficiency is in the range of 60% to 65%.

Figure 15. Spiral heat exchanger [11].

Run around heat exchanger

This heat exchanger consists of a recovery coil in the exhaust air stream connected to a preheat coil in the outside air stream and a pump which circulates the heat transport fluid (demineralised water, glycol solution, higher temperature heat transfer fluids) (Fig. 16). The heat transfer fluid receives heat from exhaust air stream which is transferred to the outside air stream. It is used to recover heat from air streams, especially when a large distance exists between the waste heat (exhaust air stream) and demand (outside air stream). The temperature of heat recovery coil is limited by the thermal transfer fluid.

T he sensible heat recovery efficiency is about (60-65)% when the temperature of exhaust stream is up to 200°C and up to 50% when the temperature of exhaust stream is above 200°C.

Figure 16. Run-around heat exchanger.

Waste heat recovery boilers

The waste heat boilers are used to recover the heat contained in the exhaust gas from turbines, reciprocating engines, incinerators, and furnaces. They consist of a certain number of parallel tubes containing water (Fig. 17). The steam generated in the tubes is collected in a steam drum from which it can be superheated or used in heating process or electricity generation. The pressure of generated steam depends on the temperature level of exhaust gases.

Figure 17. Waste heat recovery boiler.

Ion V. Ion, associate professor, mechanical engineering: “Dunarea de Jos” University of Galati, Faculty of Mechanical Engineering, Thermal Systems and Environmental Engineering Department, 111, Domneasca St., off. G102, Galati, 800201, Romania, tel. +40 740566214, e-mail: ion.ion@ugal.ro