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7.6 Desuperheaters

Desuperheaters, or recuperaters, operate by utilizing the high-pressure, superheated discharge gas temperature to heat water. The cooling of the discharge gas normally represents 15-25% of the total heat of rejection. The temperature difference between inlet and outlet gas is typically 20-50K, depending on the refrigerant and system condensation temperature (see Figure 7.21). The refrigerant gas is not meant to condense to any significant extent inside the desuperheater, so the heat exchanger should be designed to have a leaving gas temperature up to a few degrees above the condensing temperature.

The advantage of using a separate desuperheater is shown in Figure 7.22. Even if the available energy of gas desuperheating is lower than for condensing, it is possible to obtain hot water with a small heat exchanger because the "temperature pinch" is avoided. By utilizing only the sensible energy of the gas, the leaving water temperature can approach the inlet discharge gas temperature. In contrast, in a condenser the constant condensation energy makes it impossible to obtain water temperatures more than a few degrees above the condensation temperature.

Desuperheaters can be used in dedicated heat pumps to provide hot tap water, while the condensation energy is used for room heating. In a refrigeration or air conditioning system, where the condensation energy is often discharged to the ambient air or to a low-temperature water sink, the usefulness of the system can be increased by producing hot water for cleaning, sanitary purposes or for heating other processes.

The desuperheater unit can be used as a "once through" heat exchanger, where cold tap water is heated directly when it is needed (see Figure 7.23). Thus, when no hot water is needed the desuperheater will be bypassed and the condenser will reject the desuperheating energy as well. The drawback with this design is that the output of hot water is limited by the momentary capacity of the system.

If instead the desuperheater is installed with a tank for hot water, this reservoir allows a momentary use of hot water that is much faster than the direct capacity of the desuperheater (see Figure 7.24). The hot water is stored in a tank and circulated by a small pump. If the temperature in the water tank becomes too high, a valve closes and the desuperheater is bypassed.

Drainage of desuperheaters

Ideally, the desuperheater should be fitted above the condenser to allow any condensed refrigerant liquid to be drained away. This is not always possible, especially in refrigeration or air conditioning systems where the main condenser may be fitted on the roof, while the desuperheater is placed close to the compressor in the basement. This problem is even more evident with the use of glide refrigerants, such as R407C. If partial condensation takes place, the condensate has a higher concentration of the least volatile refrigerant. It is therefore even more important to ensure drainage and condensate transport to the condenser. Otherwise, there is a risk of the refrigerant components separating inside the system, resulting in unpredictable and impaired performance.

The construction of a brazed plate heat exchanger works in favor of keeping the liquid and gas phases together. In a brazed heat exchanger, the leaving droplets of refrigerant are dispersed and easily carried away by the dominant gas phase. Designing the connection pipe from desuperheater to condenser for a gas velocity of 5-10 m/s provides sufficient turbulence to avoid liquid condensate accumulation.

Scaling problems

The solubilities of calcium carbonate (limestone, CaCO3) and calcium sulfate (gypsum, CaSO4) decrease as the water temperature increases. As a result, there may be deposits of these two minerals on heated surfaces, e.g. in a desuperheater. This phenomenon is called scaling. If scaling occurs, the extra crust of mineral will have the same effect as fouling, i.e. heat transfer is impaired and the capacity of the desuperheater is decreased. Unless the water flow is adequately turbulent, there is also a risk that pockets of stagnant water will form, encapsulated by the crust. In unfavorable conditions, these pockets could act as reaction points for pitting corrosion of the stainless steel. The problem is small with soft water, but if the deposits are allowed to accumulate or hard water is used, precautions should be taken to avoid or minimize the problem.

Avoiding scaling

To avoid or minimize the risk of problems induced by scaling, such as decreased capacity, increased water pressure drop or even leakage, three parameters can be manipulated: temperature, turbulence and chemical properties.

Problems with deposits of limestone and gypsum occur only at water temperatures exceeding 65-70°C. Even if the inlet refrigerant gas temperature is much higher than 70°C, only the water temperature influences the risk of scaling. The design temperature of the water should therefore never exceed 65°C at the warmest point. Because the maximum water temperature is achieved at the heat transfer surface inside the heat exchanger, this value should be monitored in the calculation program.

If the brazed plate heat exchanger design becomes substantially over-dimensioned, due to considerations of pressure drop or other factors, it may be difficult to avoid a dangerously high leaving water temperature. In that case, it may be possible to design a desuperheater with parallel flow instead. The converging temperatures of refrigerant gas and water will efficiently limit the maximum water temperature.

Turbulent flow reduces the probability of scale, because the induced shear stresses can dislodge the newly formed deposits from the heat surface walls as harmless particles that are easily transported out of the brazed plate heat exchanger. A higher shear stress increases the efficiency of this self-cleaning effect Hence, it is important always to operate the brazed plate heat exchanger at turbulent flow. The transition point between laminar and turbulent flow in a brazed plate heat exchanger is difficult to define, because the flow constantly changes direction inside the winding channels. An absolute minimum value of the Reynolds number to achieve turbulent flow in brazed plate heat exchangers is Re = 150. A much higher Reynolds number should be used if a high outlet temperature or the quality of the water increases the risk of scaling.

If scale has formed, it is still possible to remove the minerals by the cleaning in place (CIP) method. A tank with a weak acid is connected to the circuit, and the acid is circulated with the water flow in the reverse direction (back-flush). Five-percent phosphoric acid is sufficient. If the heat exchanger is cleaned frequently, the slightly weaker 5% oxalic acid can be used. The acid dissolves the alkaline minerals, allowing them to be removed by the flow.

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