Installations of heat pumps - Aquaculture Engineering

Installations of heat pumps

When installing a heat pump location of the condenser is very important, because it is here that the energy is released. For a refrigeration plant location of the evaporator will be important because here removal of energy occurs. There are a number of places for installing the condenser in the heat pump in a fish farm; all have advantages and disadvantages (Fig. 7.14). One solution is to place it directly into the inlet water, which gives very good heat transfer. However, leakage from the condenser could lead to contamination of the inlet water by the refrigerant, which is dangerous because it could be toxic to the fish. To avoid this, an extra closed circuit is more commonly used between the con-denser and the inlet water consisting of a pump and a heat exchanger through which a non-toxic liquid (water or glycol, for instance) circulates. Glycol has a high thermal capacity so is a good choice of medium. Between 10 and 15% reduction in the COP is normal when using the extra circuit, because the heat is transferred twice.

Another method is to install the condenser in the outlet water and afterwards have a heat exchanger between the outlet water and the inlet water, also a two-step heat transfer process. The disadvantage with this method is that increased fouling will present problems. Cleaning of the condenser, which is normally of the shell and tube type, is difficult.

The evaporator is normally placed in the outlet water. Here there will also be problems with fouling, and a closed circuit can also be used to advantage in this situation. A plate exchanger is used in this circuit to transfer the heat, because it is easier to open for cleaning.

When using the evaporator in the outlet water and the temperatures are low freezing of the outlet water may be a problem because the temperature is reduced as much as possible through the evapo-rator to recover the stored energy in the outlet water, meaning that the temperature in the working medium is 0°C or less to ensure effective heat transfer.

It is therefore possible that the water in the evaporator will freeze, for instance if there is a reduction in the water flow or drop in the inlet water temperature. To avoid breakage of a shell and tube evaporator due to ice, it is normal to use glycol which functions in the same way here as in the con-denser circuit; glycol is a liquid with a very low freezing point.

In practice, there is not usually enough energy in the outlet water to get a heat pump to function with a good COP. Therefore the heat pump will usually be an integral part of a total energy system where additional energy from seawater or another low temperature source may be used. Energy may also be added directly, for instance by using an immer-sion heater; this will of course require a much smaller heater compared to using an immersion heater alone and not in combination with a heat pump.

Example

Inlet water to a fish farm is heated from 2 to 8°C by transfer of energy from the condenser. The water flow is 500 l/min (8.33 kg/s). Fifty kW of electric energy is supplied to the compressor. Find the COP for the heat pump.

Start by calculating the total energy input to the inlet water:

                     P = mc_pdt

                     = 8.33 kg/s × 4.2 kJ/(kg °C) × 6°C

                     =210 kJ/s

              210 kW

The COP is therefore

              e = 210/50 = 4.2

This means that for every kW of electric energy that is supplied to the compressor, the water is heated by 4.2 kW.