Figure 1. Latent and sensible energy percentages.
Steam contains two types of energy: latent and sensible. When steam is supplied to a process application (heat exchanger, coil, tracer, etc.) the steam vapor releases the latent energy to the process fluid and condenses to a liquid condensate. The condensate retains the sensible energy the steam had. The condensate can have as much as 16% of the total energy in the steam vapor, depending on the pressure (Figure 1).
One of highest return on investments is to return condensate to the boiler. As fuel costs continue to rise, it’s imperative to focus on recovering condensate in every industrial steam operation.
Condensate contains not only water, but also boiler treatment chemicals and the energy transferred during combustion. Condensate, therefore, needs to be returned to the boiler to:
- Improve energy efficiency
- Reduce chemical cost
- Reduce make-up water costs
- Reduce sewer system disposal costs
- Meet environmental regulations
Figure 3. A totally insulated condensate system can pay for itself in thermal efficiency.
Unfortunately, many industrial plants waste the condensate from the steam system and aren’t returning condensate to the boiler plant. Condensate being returned still loses thermal energy because of uninsulated tanks, condensate pipe, valves and fittings. The best practice for condensate systems is to insulate any device in the condensate system to prevent such losses (Figure 3).
If condensate isn’t returned to the boiler, the steam system must make up the loss with cold, untreated, raw water that has to be prepared for boiler operation. This preparation has a cost. The make-up water also contains substantially less BTU content that must be raised in the deaerator or atmospheric feed water heater. This energy addition adds even more cost to the operation. The raw water has to be chemically prepared for the boiler operation, which is an added cost.
Justification to return condensate
With high energy costs, you must return as much condensate as possible to the boiler plant for reuse. The benchmark for optimal condensate return is as high as 90%. This is possible if the plant doesn’t use direct steam injection for process applications.
Below is an illustration of the potential savings of a 44,000 lb/hr, 150 psig steam system with no condensate returned to the boiler plant:
|Average steam flow (lbs per hr)||44,000|
|Unloaded fuel cost ($ per MBTU)||15.3|
|Operating steam pressure (psig)||150|
|Steam temperature (°F)||366|
|Steam total energy (hg) BTU/lb||1195.1|
|Makeup water temperature (ºF)||55|
|Makeup water BTU content (hm) BTU/lb||23|
|Condensate return temperature (ºF)||212|
|Returned condensate energy (hc) BTU/lb||180.33|
|Benchmark fraction of condensate returned (decimal percent)||0.90|
Table 1. This is the basic data for the example calculation, which represents a typical operating steam system.
To determine potential energy losses per year, based on zero condensate returned to the boiler point, follow the calculation below:
- (hc – hm) = energy loss per lb of condensate
- (180.33 – 23) = 157.33 BTU per lb of condensate
- 44,000 lbs of steam = 44,000 lbs of condensate (90% return) = 39,600 lbs
- 39,600 lbs x 157.33 BTU per lb = 6,230,268 BTU/hr
- 6.230268 x $ 15.3 = $95.32/hr
- $95.32 x 8,760 hours per year = $835,003/yr
The potential savings is based on the energy required to elevate the make-up water to that of the condensate being returned. The calculation doesn’t take into account the savings from chemicals, water and sewer costs. It also doesn’t consider the effect of bringing condensate back at higher pressures, resulting in greater savings. The above calculation assumes no condensate is being returned to the boiler, but most industrial plants return at least a small percentage of condensate. Each plant should evaluate the cost of failing to return condensate and set forth a roadmap for returning condensate.