Gas cooling
An excellent presentation of generator gas cooling theory is to be found in (43). Major factors to be taken into consideration are the sensible heat in the gas, the water vapour content of the gas and its heat of condensation and the effects of fouling of the cooler.
Generator gas coolers come in three broad categories: natural convection coolers, forced convection coolers and water coolers.
Natural convection coolers consist of a simple length of pipe. They are simple to use and clean and require no additional energy input. They can be rather bulky, though this problem can be partly offset by using fined pipe in order to increase the conductive surface. Forced convection coolers are equipped with a fan which forces the cooling air to flow around the gas pipes. This type of cooler can be much smaller than the natural convection coolers. Its disadvantages are the extra energy input to the fan and the necessity to use gas cooling pipes of small diameters, which can lead to fouling problems. The former can in some cases be offset by using the cooling air supplied by the engine fan.
Water coolers are available in two types, the scrubber and the heat exchanger; where a water scrubber or bubbler is used, the objective is generally to cool and clean the gas in one and the same operation.
Scrubbers of many different types exist, but the principle is always the same: the gas is brought in direct contact with a fluid medium (generally water) which is sprayed into the gas stream by means of a suitable nozzle device. The advantage of this system is its small size. Disadvantages are the need for fresh water, increased complexity of maintenance, and some power consumption resulting from the use of a water pump.
The cleaning of the cooling water from phenols and other tar components is by all probability also a necessary and cumbersome operation. But so far very little experiences or cost calculations for the waste water treatment are available.
It is also possible to cool the gas by means of a water cooled heat exchanger. This is a suitable method in case a source of fresh water is continuously available and the extra investment and power consumption of a suitable water pump can be justified.