2000 Hook-up Book

Draining Temperature Controlled Steam Equipment

Case in Action: Absorption Chiller, Condensate Drainage

Solution Two Pressure Powered Pumps™ were installed in paral lel, along with necessary steam traps, air vents and strainers . The Refinery supplied the reservoir and inter connecting piping. Benefits • Regardless of varying steam supply pressure, consid ering the throttling that naturally occurs through the automatic control valve, thorough condensate drainage is assured and cooling efficiency is maintained. • Installation cost was much lower with the Pressure Powered Pumps ™ over electric pumps that were also being considered. Costly water and explosion proof control panels were not required. • Pump maintenance cost is also much lower through elimination of the need for mechanical seals and pump motors.

Absorption chillers are important sources of cooling nec essary for many refinery processes. A typical example is the need to cool products (using large heat exchangers) after the stripping process in an “alky” unit. Products going to storage are generally maintained below 100°F. Steam is used to drive the absorption process at low pressure, typically below 15 psig. Condensate drainage becomes a very real concern. In this case, steam is supplied at 12 psig to the chiller through an automatic control valve. Condensate system backpressure is a constant 6-7 psig, considering the 30 ft. uphill pipe-run to the vented condensate receiver. The Refinery Contact Engineer recognized the potential for system stall (having previously used the Pressure Powered Pump™ to overcome other similar problems).

SYSTEM DESIGN

In many cases, a fluid is heated by passing it through a series of heat exchangers which are all provided with steam through a common control valve (Fig. 48). Multiple section air heater coils or “batteries” typify such applica tions, as also the multi-roll dryers used in laundries. While the load on the first heater is usually appreciably greater than the load on later heater sections, the pro portion of the total load which each section takes is often a mat ter of “rule of thumb” or even conjecture. The temperature difference between the steam and the enter ing cold fluid can be designated ∆ t 1 . Similarly, the temperature dif ference between the steam and the outlet heated fluid can be ∆ t 0 . The ratio between ∆ t 1 and ∆ t 0 can be calculated, and will always be less than one, see Fig. 49 (page 37) If the chart at Figure 50 is entered on the horizontal axis at this ratio, a vertical can be taken upwards until the curve corre sponding with the number of heaters or coils in use is inter sected. A horizontal from this Multi-Coil Heaters

point given the proportion of the total heater load which is carried by the first section. Multiplying this proportion by the total load given the conden sate rate in this section, and enables a trap with sufficient capacity to be selected. If it is required to accurately determine the load in the second

section, estimate the temperature at the outlet from the first section, and regard this as the inlet tem perature for an assembly with one less section than before. Recalculate the ratio t s - t o /t s - ti 2 , and re-enter the chart at this value to find the proportion of the remaining load taken by the “first” of the remaining sections.

Steam Temp. t s

Air Outlet Temp. t o

Air Inlet Temp. t i

Figure 48 Multiple Coil Air Heater

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