Domestic Hot Water Recirculation Pump/Supply Sizing is the process of designing a system that ensures hot water availability at the faucets or fixtures with minimal waste of water and energy. The main components of such a system are:
- A water heater that provides hot water to the supply pipes.
- A recirculation pump that circulates the hot water through the supply and return pipes.
- A control valve that regulates the flow of the recirculation loop.
- A balance valve that maintains the pressure difference between the supply and return pipes.
The steps to design a Domestic Hot Water Recirculation Pump/Supply Sizing system are:
- Determine the flow rate of the recirculation loop based on the heat loss of the supply pipes and the acceptable temperature drop between the heater and the farthest fixture.
- Size the recirculation pump based on the flow rate and the pressure drop of the recirculation loop.
- Select the control valve based on the type of control strategy (e.g. timer, thermostat, aquastat, etc.) and the flow characteristics of the valve.
- Size the balance valve based on the flow rate and the pressure difference between the supply and return pipes.
Basic Theory
The primary goal of DHW recirculation is to minimize the time it takes for hot water to reach the fixtures. This is achieved by maintaining a constant flow of hot water through the pipes. The key parameters involved in sizing are the flow rate, pipe diameter, and pump head.
- Flow Rate (Q): The amount of water that needs to be circulated.
- Pipe Diameter (D): The size of the pipes through which the water circulates.
- Pump Head (H): The pressure required to overcome friction and elevation changes in the piping system.
Procedures
- Determine Flow Rate (Q)
- Estimate the total fixture flow demand based on the number of fixtures and their flow rates.
- Calculate Pipe Diameter (D)
- Use the Darcy-Weisbach equation to determine the pipe diameter required for the given flow rate, considering factors like material and length.
- Find Pump Head (H)
- Consider the elevation difference and friction losses in the piping system to calculate the pump head required.
- Size the Pump
- Select a pump that can provide the required flow rate and pump head.
Excel Formulas and Scenario
Let’s consider a scenario for a residential building with 10 fixtures and an estimated total fixture flow demand of 25 gallons per minute (GPM). The building has a vertical height difference of 20 feet between the water heater and the furthest fixture. The pipe material is copper, and the total pipe length is 150 feet.
- Flow Rate (Q): Q = 25 GPM
- Pipe Diameter (D): D = DarcyWeisbach(Q, Material, Length) (Excel formula)
- Pump Head (H): H = ElevationDifference + FrictionLosses (Excel formula)
Excel Table
| Parameter | Value |
|---|---|
| Total Fixture Flow Demand | 25 GPM |
| Pipe Material | Copper |
| Pipe Length | 150 feet |
| Elevation Difference | 20 feet |
Excel Formulas
- Pipe Diameter (D): D = DarcyWeisbach(Q, “Copper”, 150)
- Pump Head (H): H = 20 + FrictionLosses
Excel Calculation
Using the Darcy-Weisbach equation and other relevant formulas, we can calculate the pipe diameter and pump head in Excel. Ensure to use appropriate Excel functions and constants.
MATLAB Comparison
For comparison, let’s solve the same problem using MATLAB scripts. MATLAB provides robust numerical computing capabilities, allowing for efficient analysis of fluid dynamics and system sizing.
MATLAB Script
% MATLAB script for DHW Recirculation Pump and Supply Sizing
Q = 25; % Flow rate in GPM
Material = 'Copper';
Length = 150; % Pipe length in feet
ElevationDifference = 20; % Height difference in feet
% Pipe Diameter calculation using Darcy-Weisbach equation
D = DarcyWeisbach(Q, Material, Length);
% Pump Head calculation
FrictionLosses = CalculateFrictionLosses(Q, Material, Length);
H = ElevationDifference + FrictionLosses;
Ensure that MATLAB functions like DarcyWeisbach and CalculateFrictionLosses are defined appropriately to perform the calculations.
