# ME209 2- Compact Fluorescent Light (CFL) bulbs have been available...

## Transcription

ME209 2- Compact Fluorescent Light (CFL) bulbs have been available...
```ME209
2- Compact Fluorescent Light (CFL) bulbs have been available for years; they are very efficient, but a
little pricey. A CFL bulb that puts out as much light as a 60-W incandescent bulb might cost \$10,
compared to about \$1 for incandescent bulb. But CFL bulbs are expected to last (on average) 15,000
hours, compared to about 1000 hours for incandescent bulb. So it is easy to see that you would
need 15 incandescent bulbs (total cost \$15) to last the 15,000 hours that you would get from one
(\$10) CFL bulb; you save \$5 and a lot of climbing ladders to replace all those incandescent bulbs.
But there’s more. A CFL bulb that puts out as much light as a 60-W incandescent bulb will use about
13W of power. According to the US Energy Information Administration (www.eia.doe.gov), residential
electricity costs average about \$0.10 per kilowatt/hour (1kW/h = 3600 kW/S = 3600 kJ). Over the 15,000
hours that one CFL bulb is expected to last, how much will you save on power if you replace an
incandescent bulb in your home with a CFL bulb? Calculate the missing information in the worksheet
shown below using VBA Sub.
3- One evening, a few friends come over for a soak, and you discover that the water in the hot tub is at
115°F (46°C) – too hot to use. As your friends turn on the cold water to cool down the hub, the
engineer in you wants to know how long this going to take, so you write an energy balance on a
well-mixed tank (ignoring heat losses to the air). By integrating the differential equation of the
energy balance, you get an equation for the temperature in the tank as a function of time:
(
̇⁄ )
Where, T is the tank temperature, Tini is 115°F, the cold water temperature is 35°F and the volume (V)
and volumetric flow rate ( ̇ )are 3000 liters and 30 liters per minute, respectively.
A. Calculate the expected water temperature at any given time in minutes (user input) after the
cold water is turned on using VBA function.
B. Calculate how long it should take for the water in the tub to cool to a given temperature (user
input) using another VBA function.
```