Heat Exchanger in Ammonia Production Part II

High pressure heat exchange systems 
in Ammonia production processes

Part II

Process Gas Cooler Design

The requirement for low capital costs also impacts the design of the PGC. Design margins are squeezed out. In consequence a cost effective design is achieved, but the PGC is thermally and mechanically high loaded. All major design parameters can be seen as pieces of a puzzle, which have to perfectly fit in order to achieve reliable PGC design. Changing some design parameters has to be done carefully, since one parameter affects the other factors. This becomes more important in case of designs which are close to their limit. If the design variables are misaligned, not necessarily a sudden failure during first start-up of the PGC occurs, but life time will be reduced attended by multiple repairs, worse reliability and availability of the PGC and the overall waste heat boiler package henceforth reducing the availability of the total complex leading in higher probability of high financial losses.

Besides the thermal design goals, a mechanical design target is a tube sheet with low mechanical and thermal loading. The tube sheet is the heart of the process gas cooler. It separates the two pressure levels and media streams and is equipped with the process tubes. Two basic and well known principles of tube sheet technologies exist, namely the stiffened and flexible tube sheet, whereas the stiffened tube sheet can be distinguished in a thick stiffen tube sheet and a thin tube sheet with stiffener plates. For those concepts each specialized vendor owns its technology details. Since the process parameters and boundary conditions are preconditions, the PGC should be designed in way to reduce the impact of the process parameters onto the tube sheet, tube to tube sheet welds and the mechanical structure. In general, thin tube sheets (flexible as well as thin with stiffener plates) are beneficial, since they have lower thermal stresses compared to thick and stiffened tube sheets.

Two examples of the overall design puzzle will be discussed more in detail. In principal the dependency of the design parameters is valid for all tube sheet technologies, but depending on the tube sheet design, the situation gets more serious.

The first area of conflict between commercial and technical targets is the tube pitch at a given tube diameter and tube number. A small tube pitch reduces the area at the tube sheet, where the pressure forces by process gas and water/steam can be applied. In consequence, this leads to lower mechanical loads by the pressure forces and therefore enables for a thinner tube sheet with a positive cost effect. In addition, a smaller tube pitch leads to a smaller tube field diameter. This leads to a smaller PGC diameter and in consequence to a shell with lower wall thickness. In total, the PGC weight is lower which enables lower costs for the PGC. Also transportation from manufacturer to place of operation becomes less of a challenge.

From this point of view only, a designer should choose the smallest tube pitch. But looking into a second function of the tube pitch, this changes the assessment. The tube pitch creates channels between the tubes, in which the steam bubbles flow, which arise at the outer tube surface. From a fluid dynamic point of view a small tube pitch constraints the flow area the steam bubbles have to pass. This increases the risk that steam bubbles cannot flow off but accumulate with the consequence of dry out and therefore local over-heating. This has to be seen in conjunction with the PGC inlet heat flux, which depends on the process parameters and the tube diameter. To avoid local dry out in the tube bundle, the heat flux by design should be below the critical heat flux, which indicates the limit for local dry out. In principle, the critical heat flux depends on the pressure level of the water/steam side, tube geometry and number as well as tube pitch.

Figure 5: Variation of tube pitch at constant tube geometry and number

Figure 5 shows the principal impact of tube pitch onto the critical heat flux for a given PGC design. It can be seen clearly, that a small tube pitch leads to a critical heat flux, which is below the PGC heat flux resulting from the thermal design. Such a design has a high risk of local dry out. With increasing tube pitch, the critical heat flux also increases and is higher than the PGC inlet heat flux. This chosen tube pitch leads to reliable thermal design.

A second area of conflict refers to the ferrules, which are installed at the tube inlet over a length, which comprises the tube sheet thickness and the tube inlet. The ferrules have a double protection function. Firstly ferrules reduce the heat input by the process gas into the tube sheet and secondly, they shift the start of the steam production away from the backside of the tube sheet. The ferrules have to resist the high process gas temperatures and the fact, that for Ammonia production the process gas atmosphere at high temperatures has a high sensitivity for the Metal Dusting phenomenon, which is a special type of corrosive attack. For that reason, Nickel based alloys are used as material for the ferrules. This material is very expensive and therefore a cost reduction measure is, to decrease the length of the ferrule. A shorter ferrule still reduces the temperature input into the tube sheet, but with a shorter ferrule, the start of the steam production is closer to the tube sheet. The designer has to take care, that the steam flowing upwards to the highest point of the shell can be safely discharged through the first riser nozzle. To ensure this with a shorter ferrule means that the first riser nozzle has to be positioned closer to the tube sheet. The position of the nozzle has mechanical and code relevant constraints. The determining factor is the minimum allowable distance between the nozzle and the connecting weld of tube sheet and shell. This allowable distance can be in conflict with the required distance to ensure a proper steam discharge without risk of steam blanketing.

The above discussed situations get more serious when a thin tube sheet with stiffeners is applied. Since the stiffener plates are welded perpendicular onto the back side of the tube sheet and are between the tube rows, the available tube to tube space is reduced by the stiffener plate thickness. In consequence the available water volume around the tubes decreases and the risk of steam bubble accumulation and local dry out increases. This is intensified when short ferrules are installed and the start of steam production is in the area of the stiffener plates. To mitigate the risk of local dry out, the corrective action is to further increase the tube pitch, which enlarges the tube sheet diameter and thereby the area, where pressure forces by the media are applied. This leads to a higher mechanical loading by pressure forces of the stiffened tube sheet compared to a flexible tube sheet design.

Those two examples show clearly the interaction of design variables and the consequence of the chosen tube sheet design. ARVOS | SCHMIDTSCHE SCHACK is applying the flexible tube sheet design combined with own technology features. For extreme process conditions and requirements, a super flexible tube sheet design (SUPLEX® tube sheet) was developed and successfully applied in process gas coolers. Based on major design experience for more than 30 years, ARVOS | SCHMIDTSCHE SCHACK PGC fleet with flexible tube sheet comprises in total 10 million operating hours.