OCR Text |
Show increases the excess air flow at these low firing rates but, since the sensible heat associated with high excess air is greatly reduced, the energy penalty is minimal. Other schemes can control either the fan speed or inlet damper position to provide a constant pressure at the boiler outlet with the input signal to the controller being a function of either the fuel flow to the boilers or the pressure at the boiler flue gas outlet. There is no universal opinion as to the optimum scheme at almost every variation applied in the field. If the low level heat is being used in a closed circuit (i.e., comfort heat, low level process heating, tank farm heating, etc.), it is imperative that the return water temperature be maintained below 100°F to assure adequate performance of the heat recovery unit. This is generally accomplished using a temperature controlled three-way valve whereby if the return water temperature is greater than the set point (90-100°F), the warm water is recycled to the load to remove additional heat. In the event that the low temperature load requirement is 0, the temperature in the direct contact heat exchanger stabilizes at~150°F whereby the unit acts simply as a scrubber. If the no-load condition prevails for any substantial period of time, the flue gas should be diverted to the original stack and the circulating pump shut down. The installation of a DCCHR unit does not require any additional maintenance effort other than would be associated with its movable parts, i.e., fans, pumps. During the recent GRI equipment survey, many facilities were seen in Britain and Europe where the universal statement by boiler operators interviewed was that it's a "put it in and forget about it" piece of equipment. This statement assumes that the system has been designed and installed properly. Some units had been in operation five years with no maintenance of any kind. One of the oldest DCCHR system manufacturers in Europe used to make the heat recovery tank with a bolt-on cover to accommodate future maintenance but now sells the unit with all welded construction because they have never needed to remove the cover. 9-25 KVB1 O-P-281 |