Disposables Versus Stainless-Steel Systems
Decisions to use single-use systems should be made early in a project, typically during conceptual engineering. Single-use
systems have a large impact on the layout of a facility and also may affect automation strategies, clean utility requirements,
floor-to-floor heights, project timelines, procurement schedules, and even area classifications like heating, ventilation,
and air-conditioning (HVAC) design. Facilities that use only single-use processing sometimes realize substantial advantages
over conventional designs, but tend to be limited in scale.1 Most biopharmaceutical facilities use a mixture of stainless-steel and single-use systems and for these facilities, an analysis
of lifecycle economics can help determine the optimum mix.
Estimating Capital Costs
Capital costs for single-use systems are always lower than for conventional stainless-steel systems, so evaluating lifecycle
economics comes down to comparing differences in operating cost versus the cost of capital. In conceptual design, capital
costs are estimated from equipment cost using the Lange factor, which is an empirical multiplier that accounts for the cost
of installing the equipment. Lange factors vary depending on the type of equipment and whether it arrives as a preassembled
skid or as individual equipment items that are assembled in the field. The factor accounts for all direct costs associated
with equipment installation including setting up the equipment, utility and process piping hookups, and automation.
Table 1. Capital savings for single-use buffer bags
Table 1 shows how the Lange factor is used to estimate capital-cost savings for a simple scenario involving replacing multiple
stainless-steel buffer hold vessels with single-use bags. By doing so, not only are we able to delete the stainless-steel
buffer hold vessels, but also we are able to delete two CIP skids and their corresponding infrastructure. The single-use case
still has some capital cost (for bag holders) but its Lange factor is much lower than for stainless-steel vessels, reflecting
the bags' simpler installation. The total direct capital cost savings for this alternative are just over eight million dollars.
Table 2a. Estimated facility and HVAC costs (USD)
Often, the use of single-use systems affects either the area required in a facility or the cleanliness classification of that
area. For the above alternative, using single-use bags eliminated 1,100 square feet of grade D space, which originally contained
the stainless-steel buffer hold vessels but added back 1,075 square feet of controlled not classified space for storing single-use
bags. These changes affect both capital and operating costs that are quickly estimated from rough order of magnitude benchmarks
in Tables 2a and 2b. Experience suggests each added square foot of cleanroom space also contributes additional mechanical
space, which is included in Table 2b. Capital and HVAC costs are calculated for a base location and discounted 25% to adjust
for the local market. For this alternative, savings in facility costs are a relatively small ($58,000) with an annual savings
in HVAC costs of $791.
Table 2b. Difference in facility space costs as a result of implementation of single-use systems. Only ISO 9 (Grade D) and
controlled not classified space are affected, along with space needed in mechanical areas as a result of decreased HVAC needs
by reducing cleanroom space.