BFW Heaters

BFW Systems - BFW Heating
September 14, 2001

Introduction
Acid System BFW Systems
Blowdown Heat Recovery
Steam Drum Attemperator
Desuperheating Exchangers

Associated Links

Low grade heat in an acid plant is often rejected into the environment through the cooling water system. Energy efficient plants often employ this energy for preheating boiler feed water (BFW). There are two basic types of BFW heating that can be done:

1. Heating of undeaerated water entering the deaerator
2. Heating of BFW being delivered to the boiler (other than in an economizer)

Preheating the boiler feed water prior to entering the deaerator minimizes the use of low pressure steam in the deaerator. There are some important points to remember when specifying a system to preheat undeaerated boiler feed water.

If too much preheating is done this may cause problems in the operation of the deaerator. The supply of stripping and heating steam to the deaerator is done under pressure control. If the boiler feed water is heated to its saturation point the pressure controller may reduce or cut-off the supply of steam to the deaerator, reducing its ability to strip oxygen and carbon dioxide from the water. The exit temperature from the boiler feed water heater should not approach the deaerator operating temperature by more than 17^oC (30^oF).

Prevention and detection of leakage from the heating fluid into the boiler feed water is also required to prevent contamination of the water being feed to the boiler. Damage to a boiler can result if harmful chemicals are allowed to enter the boiler through an undetected leak.

Preheating the boiler feed water prior to entering the boiler is done to maximize steam production or to preheat the feed water to an economizers to minimize the chance of gas side condensation.

Acid System BFW Systems

In most acid plants heat from the acid system is rejected to cooling water through the acid coolers. This energy can easily be recovered by preheating undeaerated boiler feed water.

Acid circulating in the absorber acid system is generally used for BFW preheating because it is the hottest acid. The BFW heater is placed in parallel with the Absorber Acid Cooler with the acid on the shellside as in all acid coolers. Like all acid coolers the unit will be anodically protected. The Absorber Acid Cooler should be designed for the full cooling duty assuming that the BFW heater is off-line. Preheating BFW is secondary to acid temperature so the control system should be designed to maintain the desired acid temperature to the tower.

Since the BFW heater is generally install in parallel to another acid cooler, the heater should be designed for a similar pressure drop. This be dependent on the available pump discharge head but generally does not exceed 15 psi.

The boiler feed water flows through the tubeside of the BFW heaters. If possible, the system should be designed so the BFW pressure is higher than the acid pressure. This will ensure that if a leak occurs, leakage will be from the water side into the acid side, thus reducing the chance of contaimination of the BFW. At the exit of the exchanger on the water side, pH and conductivity analyzers should be installed to continuously monitor the water. Changes in pH and conductivity may indicate an acid leak into the water side. When a low pH or high conductivity alarm occurs, the operating procedure should require the operators to investigate immediately. As an additional precaution, the system can be designed to automatically dump the BFW preventing contaminated water from entering the boiler. In this case a supply of BFW (i.e. BFW Surge Tank) is required to maintain feed to the deaerator.

The pressure drop through the BFW heater is generally low. The BFW flowrate is generally much lower than the acid flowrate. When the exchanger is designed for the required heatload and shellside pressure drop, the resulting number of tubes results in a low pressure drop.

Blowdown Heat Recovery

Boiler blowdown represents energy losses and and cost of chemicals required to treat the water. Often the blowdown is simply flashed in a tank and the resulting vapour sent to atmosphere and the liquid to drain. Sometimes the vapour is recovered as low pressure steam for use in a deaerator or sent to a low pressure steam header. When quantity of boiler blowdown is high, the heat in this stream can be used to preheat the incoming fresh make-up water. To accomplish this a simple liquid/liquid exchanger is used.

Steam Drum Attemperator

A steam drum attemperator is a series of tubes submerged in the boiler steam drum. This system is used primarily to preheat the water before entering an economizer to minimize the chance of condensation in the economizer.

The BFW outlet temperature is controlled by a three-way valve which diverts BFW to the attemperator for heating. The heated BFW is mixed with the cold BFW to obtain the desired temperature. The three-way valve should fail in a position that bypasses the attemperator.

The design of a steam drum attemperator is done by the boiler vendor. BFW pressure drops should not exceed 10 psi. The steam drum may need to be increased in size to accommodate the attemperator tubes. The temperature to which the BFW is heated is dependent on the individual cases. Temperatures of 120 to 130^oC are generally high enough the prevent condensation and are not too high that the size of the economizers are excessive.

Desuperheating Exchangers

Excess superheat in the steam can be regulated by using the heat to preheat boiler feed water. The primary function in this case is to control the superheat temperature as opposed to the outlet BFW temperature. The desuperheater is simply a heat exchanger - shell and tube, U-tube, etc. - with the BFW on the tube side and superheated steam on the shell side. The pressure drop on the BFW and steam side should not exceed 10 psi. Fouling factors should be used for both the tubeside and shellside. A value of 0.001 h ft² ^oF/BTU is typical.


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