Typically the food industry produces products that need to be cooled or frozen before transport and/or consumption/utilization. On the other hand, hot water is needed for the process and for cleaning purposes. Waste heat from a refrigeration system has a temperature of 25 to 30 °C. With the use of an add-on heat pump, waste heat from the condensing side of the refrigeration system is used to heat water to temperatures up to 80 °C. The add-on heat pump will further increase the pressure of the refrigerant from the refrigeration system to achieve high condensation temperatures.
The add-on heat pump
The figure on the right shows an industrial refrigeration installation on which an add-on heat pump is installed. In this section the operating principle of the add-on heat pump is explained. Detailed information on an industrial refrigeration system is given further down this page.
Two pipes connect the add-on heat pump to the refrigeration system. The compressor of the add-on heat pump is positioned in series with the compressor of the refrigeration system. The heat pump compressor will further increase the pressure of the compressed gases from the refrigeration system. The higher pressure results in a higher condensation temperature of, for example, 70 °C. In the condenser of the heat pump, the high pressure gases will condensate and heat is transferred to the medium that needs to be heated, for example cleaning water. A condensation temperature of 70 °C results in cleaning water of 65-70 °C. The pressure of the condensed refrigerant at the outlet of the condenser is reduced and it flows back to the refrigeration system.
The discharge gases of the refrigeration system condensate at a temperature of 25 to 30 °C, however, due to compression they are superheated up to 60 to 100 °C. The superheated gases are cooled down by an intercooler before they are further compressed by the heat pump. Cooling occurs by mixing liquid refrigerant from the condenser with the superheated gases. Because the gases are cooled, compression will be much more efficient. Moreover, discharge gas temperatures of the heat pump compressor are not too high. Too high temperatures may demolish compressor oil.
The capacity of the heat pump compressor is controlled by the heat demand of the cleaning water flowing through the condenser. The capacity and thus the amount of compressed gases flowing through the heat pump compressor varies due to this control system. The refrigeration condenser takes care of condensation of the remaining compressed gases. Herewith only the required heat is processed by the add-on heat pump.
The refrigeration system
The figure on the left shows the type of refrigeration installation that is frequently used in the industry. The separation vessel contains fluid as well as gaseous refrigerant. The fluid refrigerant is circulated over evaporators with the use of a pump. The refrigerant temperature is lower than the process that needs to be cooled. Therefore, the refrigerant is able to extract heat out of the process at the evaporator. The heat causes the refrigerant to partially evaporate. The gas/liquid mixture is than led to the separation vessel. To maintain a constant pressure level inside the vessel, gas is extracted from the separator by the compressor. Due to the compression, both the pressure and the boiling point of the refrigerant are increased. Due to this higher boiling point compressed gasses can release heat towards their environment. This heat release takes place inside the condenser of the refrigeration system. Because of the heat that is removed the refrigerant will condensate (become fluid). The pressure of the fluid refrigerant is than lowered and it is transported back to the separation vessel. Hence, yet another cycle can be started.
The amount of waste heat that is released at the condenser side is almost equal to the heat that is extracted from the product that needs to be cooled. The difference is additional energy due to electric energy used for the refrigeration compressor. In general, the waste heat extracted at the condenser side has a temperature that is too low to be useful. Application of an add-on heat pump gives the opportunity to efficiently upgrade the waste heat to a useful temperature level.