As pointed out by Douglas Kosar in his article, "The Answer is 3" which appeared in the July issue of Engineered Systems (page 60), the "ventilation load index" or VLI, is a fairly new term that is gaining popularity in our industry. It was introduced to emphasize the difference between the sensible and latent ventilation loads (in ton-hrs/cfm) of outside air introduced inside to a space neutral condition (75 degrees F/50% rh).
"The Weather Report" has adopted VLI terminology with a twist. The reported cooling VLI in this report is calculated using 55 degrees instead of a space neutral condition in order to identify the cooling loads "at the coil." Using a base of 55degrees is more useful in calculating ventilation energy consumption because it is commonly used as a supply air temperature setpoint in the cooling mode.
Creating Your Own Heat Recovery Savings Calculator (Continued)
Last month, in Steps 3, 4, and 5, we created the "report" worksheet that is designed to present the results as well as pertinent project information in an organized report. We also developed a psychrometric calculator that calculates enthalpy and humidity ratio of the design indoor space conditions for any altitude.In this issue, we will continue developing the report worksheet. As always, if you would like to follow along and still have not obtained a copy of the Boston text file, just drop me a note and I'll send you a copy of the text file.
Step 6 - Winter and summer space design humidity ratio and enthalpy (input blocks #2 and #3). If you are using the psychrometric calculator, input the formulas given in Figure 3 into the designated cells. If you are manually inputting these values, leave the cells unshaded to indicate that they are required inputs. Also the humidity ratio should be expressed in grains/lba in order to be consistent with the hourly data. So, change the units from lbw/lba to grains/lba as shown.
Step 7 - Thermal effectiveness. Create the thermal effectiveness input block #4 as shown in Figure 2. Thermal effectiveness is defined as the ratio of the actual transfer of energy or moisture to the maximum possible transfer between airstreams. The effectiveness is a function of the heat exchanger construction including size, materials, flow path, and configuration. This value can vary widely for different types of heat recovery systems.
For manufactured heat recovery systems, the sensible or total energy recovery effectiveness is available from the manufacturer. For engineered systems like runaround coil loop systems, the effectiveness can be calculated using a coil selection program via trial and error or could be assumed. Figure 4 provides some additional guidance.
For a more in-depth overview, refer to 1996 ASHRAE Systems and Equipment, Chapter 42. For now, let's assume a total energy heat exchanger (desiccant wheel) operating at 75% efficiency. So, type in 0.75 for both sensible and total effectiveness as shown.
Step 8 - Operating schedule. Create the operating schedule input block #5 as shown in Figure 2. This will allow you to define when the heat recovery system (and associated ventilation and exhaust systems) is on or off. Input the desired hour (from 1 through 24) when the system will start and stop on weekdays and weekends. For now, let's go with starting at 8 (8:00 a.m.) and stopping at 18 (6:00 p.m.) for both weekdays and weekends. See you next month! ES
EDITOR'S NOTE: The images associated with this article do not transfer to the Internet. To review the figures, please refer to the print version of this issue.
