Lacey (1993) has written about the selection of mechanical systems for laboratories. He describes the process in general and refers to a base case in particular which provides an exacting review of the design considerations, economic factors, and performance evaluations of VAV vs. CV systems.
Design heat gain. Design heat gain in a laboratory is the total cooling load resulting from equipment, lighting, people, insulation and conduction. The design engineer must calculate this load using information supplied by the owner and experience with previous lab buildings.
The 1989 ASHRAE Handbook of Fundamentals states that heat gain from equipment commonly ranges from 15 to 70 Btu/h. ft2 (50 to 250 W/m2). It also points out that this can be as much as four times the heat gain from all other sources. Chapter 14 of the ASHRAE Handbook HVAC Applications includes a table of recommended heat gain figures for various pieces of equipment. This is a valuable guide in designing a specific laboratory room but often the future equipment of a new facility is unknown. The laboratories of a new high technology building may hold devices (such as DNA synthesizers) that were not commercially available when design of the facility began.
Three factors lead designers to overestimate the peak heat gain.
The economic response of VAV [steady] and constant volume [rising] systems to design heat gain is very different.
In the variable flow system design heat gain is used to size the VAV supply boxes to each room and, with diversity, the supply duct work and air handlers. In operation, room flow will modulate to meet the greater of hood flow or required cooling flow. The operating costs are insensitive to design heat gain.
The impact on constant volume systems is very different and extremely dramatic. At low design heat gains (less than 10 W/ft2 (108 W/m2) for the base case, design room flow will be determined by the exhaust need of the fume hood. In [this situation], installation cost and operating cost are insensitive to design heat gain. More commonly, design heat gain will exceed 10 W/ft2 (108 W/m2), and the flow requirement for peak heat gain will be greater than hood flow. In this condition, operating cost is directly related to design heat gain.
Maximum hood flow
The maximum hood flow is the product of hood width, maximum sash opening, design face velocity and the number of hoods per laboratory. As one would expect, the maximum hood flow is a very strong factor for both constant volume and VAV system cost. Hood face area is the mathematical product of hood width, sash height and the number of hoods per laboratory. If hoods are very small or few in number, heat gain in the laboratories will determine the required air flow and cost is insensitive to hood size. In the base case [example], face velocity is 100 fpm (0.5 m/s) and the cost break occurs at a hood width of 3 ft (1 m) for VAV systems and 6 ft (2 m) for constant volume systems. When hoods are larger than these threshold widths, operating cost is essentially proportional to maximum hood flow.
Climate
Sites throughout North America experience a very wide range of climate. The amount of energy required to condition outside air to a supply temperature also varies. In [the example] analysis, climate is quantified as the number of heating and cooling degree days based on 55 °F (13_ C). This value can be derived from a bin analysis, or the computations presented by Erbs [1983] can be used to shift degree days values based on other temperatures. In climates where the 55°F (13°C) base heating degree days exceed 3,000, the energy saved by the heat recovery system is enough to result in a system life cycle cost that is lower than a constant volume system without heat recovery.
Heating cost
The cost of heating fuel varies greatly between laboratory facilities. Large complexes with district heating and cooling systems may elect to examine alternatives using only the marginal fuel cost, which can be $3 per million Btu or less. In contrast, owners without central utility systems might pay up to five times this amount for purchased energy.
Cooling cost
The impact of cooling energy cost is much less severe than heating energy for the base case [example]. This is because all systems included a supply temperature of 55°F (13°C), with reheat coils at each room. Less energy was required for cooling ambient air to this temperature, on an annual basis, than to preheat.
VAV control cost
This [example] model includes hood face velocity controls with the VAV system that are capable of varying hood flow in response to sash position. There are several manufacturers of equipment that will control the variable volume air flow through a laboratory, and this is an increasingly competitive field. Controls should be selected that have proven dependability. Unit cost should be considered secondary. This [example] case included an allowance of $4,000 per hood for VAV controls and boxes. However, this cost could increase to nearly $10,000 before the life-cycle cost of VAV was challenged by a constant volume system. (Costs for these systems change constantly. The designer is advised to obtain most current costs.)
Heat recovery cost
The installation of a heat recovery system also requires additional equipment cost. As with VAV components, the cost of heat recovery requirement does not strongly affect life-cycle cost. However, because the life-cycle costs of constant volume systems with and without heat recovery are very close, the cost of heat recovery equipment can be enough to determine which is not desirable.
Interest rate
Systems that reduce energy consumption usually require increased capital investment in anticipation of future savings. This is the case for laboratory buildings that include VAV or heat recovery systems. Such investments are sensitive to interest rates, although the impact is less than most other factors.
Service Life
Between universities and development industries, there is a very different perspective of a facility's functionality. This model [example] includes only the incremental first cost of VAV and heat recovery systems above a constant volume building. The building structure and basic mechanical system are common to all [buildings] and are not included in the life-cycle cost analysis. The service life is defined not as the life of the structure, but as the operating period between major renovations of the facility. As with the interest rate, constant volume systems are not affected. The life-cycle costs of VAV and heat recovery systems show moderate, inverse relationship to increased service life.
Hood diversity
Hood diversity is defined as the average sash opening for all hoods in a facility as a percentage of the full open height. Hood diversity has also been defined by some as the percentage of hoods in use. However, that definition may not properly quantify air flow through a VAV building. Generally, VAV hoods are always on at some flow and the amount of exhaust is determined by sash height. Modern VAV hood controls modulate air flow to track sash height independent of activity in the hood. This is the most important design [aspect] for variable volume systems because a system that controls face velocity will reduce the volumetric flow as sash height is lowered.
Air flow through the room (and the cost to condition outside air) reduces proportionally until the room flow meets the requirement for space cooling. The effect of this parameter in a VAV system in extreme. If all sashes are full open, the building flow is equal to that of a constant volume facility, with no savings realized from VAV equipment that controls variable flow. With all sashes closed, the building behaves like a VAV system in a dry laboratory or office building, and flow then responds to heat gain in the space.
[In one example] [T]he break-even point for VAV occurred in the base model at a diversity of 70%. At higher hood use factors, constant volume offers the lowest life-cycle cost. In an operating facility, it is certain that the average sash height will not be 0% or 100%. However, very little has been published to assist designers with an informed estimate. In any particular project, the designer must estimate how cooperative the user group will be in keeping sashes as low as possible. This will depend on the group's familiarity with variable volume systems and the ability of the facility operator to solicit their support. It is expected that the users' behavior will not be consistent between private facilities and academic institutions because private facilities can maintain tighter control of hood use.
In a striking contrast, the economics of constant volume systems are insensitive to hood diversity. In these laboratories, room flow will be constant, with exhaust air moving through the hood face, through a bypass grille or through a room exhaust grille. In reality, the flow of a constant flow or bypass hood is not constant and varies somewhat with sash position. This variation has not been considered in this model.
Average heat gain
Average heat gain is the cooling load, on average, that is realized in the operating laboratories. Surprisingly, the life-cycle operating costs of constant and variable volume systems are not highly sensitive to this factor.
Generally, the air flow required for fumehood exhaust exceeds that necessary for cooling. In the unusual cases where low hood diversity (less than 25 percent) or high average heat gain (above 8 W/ft2; 90 W/m2) occur, VAV systems are sensitive to average heat gain.
The operating cost of constant volume systems [with reheat] is actually inversely related to average heat gain, because higher space gain results in less reheat energy. The reduction in system energy for reheat is offset by increased electrical consumption by equipment in the laboratory, which has not been included in this [example].
Maintenance
Increased maintenance cost due to active components of a VAV or heat recovery system must be included in the life-cycle analysis. While this cost must be fully anticipated by facility operators, it has only a small impact on the life-cycle cost of either system.
VAV failure rate
Most VAV systems are designed to provide full flow if a component of the control system fails. In a very large facility, it is expected that some percentage of the hoods will be in a failed mode at any time. One study found that nearly 20% of the observed sample had failed and were operating as constant volume hoods. The impact of this characteristic can be included in the life-cycle cost analyses of VAV systems. However, it is found to have a very minimal effect.
Heat recovery effectiveness
The effectiveness of a heat recovery system is defined as the ratio of the recovered energy to the total energy difference between the exhaust and supply air streams. For this [example], the energy penalty for auxiliary requirements such as increased pump and fan energy are incorporated in the overall effectiveness factor.
Cost sensitivities
One interesting way to examine the impact of various factors on system cost is to graph the life-cycle cost while varying one parameter at a time. [T]he sensitivity of cost in a VAV system [is related] to five parameters.
[D]iversity is the [most sensitive] factor. Also, at some low value, diversity has no impact because the flow requirement for cooling has taken control. This break occurs at about 0.5 times the base [example] value, or a diversity of 25%. Such breakpoints in a sensitivity curve are important in establishing when various laboratory factors will drive the cost function.
The sensitivities of a constant volume system are somewhat different than for VAV systems. Annualized cost is insensitive to peak heat gain at low values where hood exhaust need will determine flow rate. Above a breakpoint of about 0.9 to 1.0 W/ft2 (108 W/m2), design heat gain becomes the most dominant action. Constant volume systems are not affected by hood diversity and are more sensitive to fuel cost than climate. This is because reheat will be necessary in any climate in addition to preheat energy.
Many other laboratory design and operating factors have much less impact. It is interesting to note that on any particular project, the design team can do nothing to change climate and energy cost, and little is documented regarding hood use and peak heat gain.
