Counterflow Versus Parallel Flow

The  superior  efficiency  of  Clayton  Steam Generators,  compared  to  conventional  fire-tube boilers,  is  a  result  of  the  counterflow  coil  design, which is made possible by forced water circulation.

The  following  illustration  shows  how  the  monotube counterflow design functions

Note that the exhaust gas  leaves  the  portion  of  the  heating  coil  that  has the  lowest  temperature  fluid  --  the  feedwater.  For this  reason,  the  exhaust  temperature  at  low  fire rates is actually lower than the steam temperature. It should be pointed out that the stack temperature of the  Clayton  Steam  Generator  is  limited  by  the feedwater  temperature.  Stack  temperature  in  the conventional boiler is limited to steam temperature.

In  operation  stack  temperature  is  an  indication  of relative efficiency. The lower the stack temperature, the  higher  the  efficiency  --  assuming  that  other conditions  such  as  CO₂,  O₂  and  radiation  losses remain  equal.  At  partial  loading  the  stack temperature  is  lower  because  of  the  proportionally greater  heat  transfer  surface,  e.g.  at  50  percent load,  the  heating  surface  per  BTU  transferred  is twice  that  at  100  percent  load.  The  stack temperature  can  only  approach  the  temperature  of the  heating  surface  at  the  point  of  exit  of  the  flue gases:  steam  temperature  for  the  fire-tube, feedwater  temperature  for  the  Clayton  Steam Generator. Because boilers operate at less than full rated load most of the time, partial load efficiency is more important than full load performance.

Figure  2  shows  a  graph  of  the  typical  stack temperatures for a conventional boiler and a Clayton Steam  Generator,  each  at  14  bar  steam  pressure. The  graph  shows  the  dramatic  difference  between the two temperatures at the lower firing ranges. Both types  have  essentially  equal  temperatures  at  100 percent rating.  
At 50 percent rating the Clayton unit is 60˚C lower. At 20 percent rating the temperature is 85˚C lower. The lower portion of the graph shows the difference in  temperatures  and  emphasizes  the  difference  in the typical load range of 30 to 70 percent.

Figure  3  illustrates  typical  efficiencies  for  Clayton Steam  Generators  and  fire-tube  boilers  at  all  loads from  0  to  100  percent  rating.  Note  that  for  the Clayton  unit  the  curve  is  higher  at  the  lower  firing rate than it is at 100 percent. This is partly due to the increasing  ratio  of  heating  surface  to  fuel  input, partly due to relatively small heat radiation loss, but most importantly because of the counterflow coil that is unique to the Clayton design.

At full load the efficiencies are virtually identical, for competitive  reasons.  With  the  fire-tube  boiler, efficiency  falls  off  at  the  lower  rates  because radiation loss is constant and therefore becomes an increasing detriment at low firing rates.