Conclusions | Table of Contents | Financial Calculations

Engineering Calculations

Natural Gas-Fired Water Heater

Load Factor

Below is a calculation of the load factor of the facility’s natural gas-fired water heaters. ACN has 2 water heaters. Each heater has an energy input, Q, of 1 MBtu/hr.

Assume:

1. TH = 120 ° F

2. TC = 64 ° F

3. VW = 20,000 gal/day for both heaters, i.e., 10,000 gal/day each

4. hWH = 0.70

Solve for LF:

The unit conversion 15.0 hr/day comes from the average number of business hours per day.

This translates to 6.68 hours of runtime per day for each water heater.

Capacity

Below is a calculation of how much hot water the water heaters are capable of producing. Each water heater holds 500 gallons of hot water. If the water heaters run during all business hours, i.e., 15.0 hr/day, calculate VW for each heater as follows:

or

or

For two heaters, this translates to 50.2 gal/min of capacity, or as shown above 45,100 gal/day. The two heaters exceed the water heating requirements of the health facility.

Natural Gas Usage Distribution

The monthly gas usage records2 and the properties of the facility’s equipment were used to determine an approximate distribution of their gas usage. According to the records provided by the Atlanta Gas Light Company, the significant changes in monthly natural gas usage occur between May and June for the transition from winter to summer and between October and November for the transition from summer to winter2.

According to the utility records2, the average summer gas usage at ACN was 4615 ccf per month, which is equivalent to:

The average winter gas usage was 8251 ccf/mo, or 275.0 ccf/day. The total annual, or overall average gas usage was 6736 ccf/mo, which is 224.5 ccf/day.

Dryer

The thermal energy input to the dryers and the heating value of natural gas were used to find out how much natural gas the dryers use. The thermal energy input to a dryer is 180,000 Btu/hr. The heating value2 of natural gas is 102,800 Btu/ccf.

Assume:

1. Average cycle length is 40 minutes.

2. An average of 60 towels are dried in each cycle.

3. An average of 900 towels are dried each day. This assumption is based on an approximate average number of people who attend the club each day, each using one towel.

Washer Water Heat

Below is an estimate of how much natural gas is used for the washing machines. The washing machines use 120 gallons of water for each cycle, 70% of which is heated to 120 °F.

Assume:

1. Approximately 80 towels are washed in each cycle.

2. hWH = 0.70

3. TC = 63.8 °F

Shower Water Heat

The water usage records4 were used in conjunction with the natural gas usage records2 to determine how much gas is used to heat shower water. Approximately 20,000 gallons of water are consumed every day at ACN. To find how much of that water is used in the showers, subtract the amount used in the washing machines.

washer water consumed

shower water consumed

The temperature rise of the water, DTW, was approximated as 40 °F.

Spa and Pool Heat

The amount of Spa and Pool Heat natural gas consumption was assumed to be the remainder of the average summer usage.

VNG = 153.8 - (17.5 + 6.14 + 86.2)

VNG = 44.0

Building Space Heat

For the calculation of Building Space Heat, it was assumed that the Overall Building Space Heat was the remainder of the overall average usage and likewise for the Winter calculation. No Building Space Heat is needed during the summer months.

Summer: 0%