Heat Exchanger Design Example
Assume that a building with a design heating load of 50,000 BTUH at 13°F winter design outdoor temperature, and a cooling load of 36,000 BTUH at 96°F summer outdoor temperature. The load profile would look like this.
This profile can be reduced to two algorithms, for use in a computer spreadsheet that can be setup to do the calculations. Similar equations can be developed for the capacity and energy use of the ground source heat pump and the competitive equipment. In some cases, a load profile is developed for both occupied and unoccupied building hours. If so, the bin temperature data can then be allocated accordingly.
In addition, the loss of efficiency due to partial load operation, when the equipment cycles off and on, must be considered. This cycling increases the run time for the equipment. This adjustment is called the PLF - partial load factor. PLF is defined as the Theoretical Run Time divided by Actual Run Time. Studies have resulted in an analytical expression for PLF which uses a "degradation" factor called CD [C sub D], as shown here.
CD has an assumed value of about 0.25 and the PLF can not exceed 1.0. The actual run time is the theoretical run time multiplied by the PLF.
The bin calculation can then be continued. The use of a personal computer and a spreadsheet program such, as Lotus 123 or Excel, can facilitate all these calculations.
You can approximate the entering water temperature of the ground-source heat pump. Typically the ground water temperature will vary from the minimum during the cold months to the maximum in late summer. A good starting estimate of the minimum water temperature is 40°F above the coldest outdoor temperature or 25°F. whichever is the greatest. A good starting estimate of the maximum water temperature is 10 to 20°F below the highest outdoor temperature at the location.
During the spring and fall, the ground temperature is near its mean value and the entering water will be about the same because the heat pump operates infrequently. Equations can be written to approximate the entering water temperature to the outdoor air temperature.
Ground load calculations for the design of the ground heat exchanger requires information on the amount of heat transferred to or from the ground during specific time periods.
During the cooling season the ground load is the sum of:
- The cooling load in unit BTUH capacity times bin hours,
- The unit power input in unit kWs times the actual run time percent times the bin hours, and
- The loop circulating pump power in kWs times bin hours.
The ground load can be calculated for the year and for the peak heating and cooling months. This information is then used to design the ground loop heat exchanger.
The IGSHPA Installation Guide gives complete details, sample calculations, and develops various equations.