Question 17.3: Heating Coil Selection The heat load for a space to be heate......
Heating Coil Selection
The heat load for a space to be heated by a forcedair heating coil is 20,000 Btu/h (5,860 W) in a building at an altitude where the air density is 80% that at sea level. The space is maintained at 70°F (21°C), and hot water is supplied from the heating plant at 180°F (82°C). Select a coil from Table 17.2 for this application. For proper air distribution in the space, the airow rate must be at least 500 ft³/min (236 L/s).
Given: \dot{Q} = 20,000 Btu/h, T_{db,i} =70°F, T_{w,i} = 180°F, \dot{V}_{a} = 500 ft^{3}/min (236 L/s)
Find: Coil water flow
Lookup values: See Table 17.2.
| TABLE 17.2 | |||||||||||||
| Example of Manufacturer’s Two-Row Heating Coil Capacity Table | |||||||||||||
| Capacity, kBtu/h | |||||||||||||
| Air Flow Rates, ft³/min | |||||||||||||
| Water Flow Rate, gal/min |
Water Pressure Drop, ft |
200 | 250 | 300 | 350 | 400 | 450 | 500 | 550 | 600 | 650 | ||
| 0.3 | <0.1 | 12.6 | 13.4 | 13.9 | 14.4 | 14.7 | 15 | 15.2 | 15.4 | 15.5 | 15.7 | ||
| 0.5 | 0.1 | 17.1 | 18.8 | 20.2 | 21.3 | 22.3 | 23 | 23.7 | 24.3 | 24.8 | 25.3 | ||
| 0.8 | 0.2 | 19.3 | 21.6 | 23.5 | 25.1 | 26.4 | 27.6 | 28.6 | 29.5 | 30.4 | 31.1 | ||
| 1 | 0.3 | 20.6 | 23.3 | 25.5 | 27.4 | 29 | 30.5 | 31.8 | 33 | 34 | 35 | ||
| 1.5 | 0.5 | 22 | 25.1 | 27.8 | 30.1 | 32.2 | 34 | 35.6 | 37.1 | 38.5 | 40 | ||
| 2 | 0.9 | 22.8 | 26.2 | 29.1 | 31.7 | 34 | 36 | 37.9 | 40 | 41.2 | 42.6 | ||
| 2.5 | 1.3 | 23.3 | 26.8 | 30 | 32.7 | 35.1 | 37.3 | 39.4 | 41.2 | 43 | 44.5 | ||
| 3 | 2.6 | 23.6 | 27.3 | 30.5 | 33.4 | 35.9 | 38.3 | 40.4 | 42.54 | 44.2 | 45.9 | ||
| (T_{w,i} − T_{db,i} ) | 160° | 150° | 140° | 130° | 120° | 110° | 100° | 90° | 80° | 70° | 60° | 50° | 40° |
| Factor | 1.10 | 1.03 | 0.97 | 0.90 | 0.83 | 0.69 | 0.69 | 0.62 | 0.55 | 0.48 | 0.41 | 0.34 | 0.28 |
Source: Courtesy of Anemostat, Inc., Scranton, PA.
These data are based on an (T_{w,i} − T_{db,i} ) value of 145°F. For other (T_{w,i} − T_{db,i} ) values, use the correction factor listed on the last row of the table. T_{w,i} is the entering water or stream temperature and T_{db,i} is the entering dry-bulb temperature. The data applies to sea level operation
The heat rate must be adjusted by two factors before the table can be used. First, since the temperature difference is 110°F, not the standard 145°F used for the tabular values, the heat rates in the table must be multiplied by the factor 0.76, read from the bottom of Table 17.2. Second, the heat rate is 20% less than that at sea level due to the high altitude of the site.
Hence, the value of heat rate to be read from the table is
In the table under the 500 ft³/min column, one finds a heat rate of 35.6 kBtu/h corresponding to a water flow rate of 1.5 gal/min. A higher airow rate, say 550 ft³/min, could be used. For this airflow, one finds a heat rate of 33 MBtu/h at a lower water flow rate of 1 gal/min. Which operating value is selected from the table ultimately depends on the airflow or water flow available at the location of this terminal heating unit and the operating costs.
Comments
When using tables for coil selection, the designer should always pick a value equal to or larger than the heat load. This provides a small capacity margin if the coil specified should fall below specifications. The water-side pressure drop shown in the second column of Table 17.2 is needed for piping systems design. Recall that the larger the product of pressure drop and flow, the larger the pump operating energy needed. The same rule applies for fan energy use; if the 550 ft³/min coil is used, the fan energy will increase by the cube of the flow rate (see the discussion of fan laws in Section 16.3.4)