CPD Test Prep
Circulation Systems
(mini test answers)
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1. When calculating heat losses in a domestic hot water system, which part of the system's heat loss is typically ignored to find the gallons per minute (gpm)?
a) Heat loss from the water heater
b) Heat loss from the supply mains
c) Heat loss from the return lines
d) Heat loss from the fixtures
Answer: c) Heat loss from the return lines
Explanation: When calculating the gallons per minute for circulation in a hot water system, the heat loss from the return lines is typically ignored. This is because the main focus is on maintaining the required temperature in the supply mains and risers.
2. What is the formula to calculate the required circulation rate in a domestic hot water system?
a) Total Btu loss / (500 x ∆T)
b) Total Btu loss x ∆T
c) Total Btu loss / ∆T
d) 500 x Total Btu loss / ∆T
Answer: a) Total Btu loss / (500 x ∆T)
Explanation: The required circulation rate is determined by dividing the total heat loss (in Btu) by the product of 500 and the temperature differential (∆T). This formula calculates the flow rate in gallons per minute needed to compensate for the heat loss.
3. In a pumped circulation system, what is the recommended temperature differential range for designing the system?
a) 5°F to 10°F
b) 10°F to 20°F
c) 20°F to 30°F
d) 30°F to 40°F
Answer: b) 10°F to 20°F
Explanation: A temperature differential of 10°F to 20°F is typically recommended for designing a pumped circulation system. This range provides a balance between efficient heat transfer and reasonable circulation rates.
4. Why is the static height of the piping above the pump not considered when determining the recirculation flow rate?
a) Because it does not affect the temperature
b) Due to the negligible impact on flow rate
c) Because the pump compensates for the height automatically
d) Static height is always considered in calculations
Answer: b) Recirculation systems are closed systems
Explanation: The static height of the piping above the pump is generally not considered when determining the recirculation flow rate because the pump is operating in a closed system which introduces static regain.
Static regain refers to the phenomenon where the pressure head gained by water flowing downhill in a pipe can be partially recovered as it flows back uphill. In the context of a recirculation system:
- As hot water circulates through the loop, it reaches fixtures at higher elevations (greater static height).
- When it's not being drawn at a fixture, this water flows back down to the pump due to gravity.
- As it descends, the water gains pressure head due to the static regain effect.
5. Under what conditions is the recirculation pump in a domestic hot water system not needed?
a) During the night
b) During periods of little or no system draw
c) When the distance from water heater to last fixture is less than 50 feet.
d) During peek hours
Answer: c) When the distance from water heater to last fixture is less than 50 feet.
Explanation: A recirculation pump in a domestic hot water system is not needed when a water heater is less than 50 feet from the furthest fixture. Typically in commercial buildings recirculation pumps use timers to turn the pump off at night, but the answer is not specific to the building type. In a hotel hot water is circulated throughout the night. Same can be said for answer d, as during most peek hours there will be maximum system draw, however during peek hours there may still be need for circulation depending on hot water demand of that particular day.
6. What is the primary objective of designing a hot water circulation system?
a) To minimize water usage
b) To ensure hot water is readily available at any fixture as needed
c) To reduce the cost of water heating
d) To increase the water pressure at outlets
Answer: b) To ensure hot water is readily available at any fixture as needed
Explanation: The main objective of designing a hot water circulation system is to have hot water readily available at any fixture when needed. This minimizes the waiting time for hot water and enhances user convenience.
7. What alternative method can be used to maintain water temperatures in the piping system?
a) Installing additional insulation
b) Heat tracing of the piping
c) Using larger diameter pipes
d) Increasing water flow rate
Answer: b) Heat tracing of the piping
Explanation: Heat tracing is an alternative method for maintaining water temperatures in the piping system. It involves running electrical heating elements alongside or around the pipes to compensate for heat losses and keep the water at the desired temperature.
Going on a Tangent
While the terms "heat trace" and "temperature maintenance cable" are often used interchangeably, there are some subtle differences between them:
Heat Trace:
General term: "Heat trace" is a broader term encompassing various types of electrical cables used to provide heat and prevent freezing in pipes, vessels, and other equipment.
Applications: Primarily used for preventing freezing in pipes for plumbing systems, industrial equipment, and outdoor applications.
Temperature control: May not offer precise temperature control, often focusing on maintaining a minimum temperature above freezing.
Types: Includes self-regulating, constant wattage, and mineral insulated cables.
Temperature Maintenance Cable:
Specific purpose: This term typically refers to a more narrowly defined type of heat trace cable specifically designed for maintaining a consistent temperature range over extended periods.
Applications: Used in industrial processes, medical equipment, food and beverage storage, and other applications requiring precise temperature control.
Temperature control: Offers superior temperature regulation capabilities, often with the ability to maintain specific temperature setpoints within a narrow range.
Types: Often self-regulating cables or cables with external thermostats and control systems.
In summary:
Heat trace: broader term for cable that prevents freezing.
Temperature maintenance cable: specific type of heat trace for precise temperature control.
8. Why should hot water velocities be limited to 6 fps in copper pipe?
a) To reduce noise in the piping system
b) To prevent water from eroding the pipe from the inside out
c) To avoid excessive pressure buildup
d) To minimize heat loss
Answer: b) To prevent water from eroding the pipe from the inside out
Explanation: Limiting hot water velocities to 6 fps in copper pipes is important to prevent erosion corrosion. Higher velocities can cause increased wear and tear from the inside, potentially leading to leaks and pipe failures.
9. What are the three basic factors that govern the rate of circulation and the size of the circulating piping in a hot water distribution system?
a) Pipe diameter, water pressure, and temperature
b) The heat loss rate of the piping, temperature differential, and allowable friction head loss in the piping
c) Pipe material, length of piping, and number of fixtures
d) Water flow rate, pipe insulation, and ambient temperature
Answer: b) The heat loss rate of the piping, temperature differential, and allowable friction head loss in the piping
Explanation: The rate of circulation and the size of circulating piping are determined by the heat loss rate of the piping, the temperature differential the system is designed to maintain, and the allowable friction head loss in the piping. These factors ensure efficient and effective circulation of hot water.
10. What is the first step in the sizing procedure for hot water supply and circulating piping?
a) Determining the water pressure requirements
b) Selecting the pipe material
c) Determining the heat loss of the hot water supply and circulating piping
d) Calculating the total water flow rate
Answer: c) Determining the heat loss of the hot water supply and circulating piping
Explanation: The initial step in sizing hot water supply and circulating piping involves calculating the heat loss from the pipes. This is crucial for understanding how much heat needs to be added to the system to maintain the desired water temperature, influencing the sizing and design of the entire system.
Step-by-Step Guide for Sizing a Circulating Hot Water System
Step 1: Assess Heat Loss of Piping
Action Steps:
- Calculate the heat loss from the hot water supply and circulating piping. Utilize heat loss tables or calculations specific to your piping material, size, insulation, and environmental conditions.
- Focus primarily on the heat loss from the supply mains and risers.
Tips:
- Accurate heat loss calculation is crucial for the efficiency of the system. Ensure you have the correct data for pipe material, insulation quality, and ambient temperature.
- Utilize reliable references or software for heat loss calculations to enhance accuracy.
Step 2: Calculate Required Circulation Rate
Action Steps:
- Apply the standard formula: Total Btu loss / (500 x ∆T) to calculate the circulation rate.
- Choose a ∆T (temperature differential) that is practical for your system, typically between 10°F to 20°F.
Tips:
- The chosen ∆T impacts the system's efficiency and operational cost. A lower ∆T might increase the circulation rate but can also increase the energy cost.
- Consult with a heating engineer if unsure about the optimal ∆T for your specific application.
Step 3: Determine Appropriate Piping Sizes
Action Steps:
- Based on the calculated circulation rate and heat loss, determine the size of the circulating piping.
- Ensure the pipe sizes are neither too large (which can lead to unnecessary heat loss and cost) nor too small (which can restrict flow and efficiency).
Tips:
- Oversized pipes increase the cost and thermal loss; undersized pipes may not deliver hot water effectively.
- Use pipe sizing charts or software to aid in selecting the correct pipe sizes.
Step 5: Select Appropriate Pump
Action Steps:
- Choose a pump that matches the flow rate and head loss requirements of your system.
- Ensure the pump’s capacity aligns with the design flow rate and can handle the head loss effectively.
Tips:
- Selecting a pump that is too powerful can lead to energy inefficiency, while an underpowered pump may not provide adequate circulation.
- Consider the future expansion of the hot water system when selecting a pump to allow for potential increases in demand.
By following these steps and considering the provided tips, you can effectively size a circulating hot water system that meets the specific needs of your building while ensuring efficiency and cost-effectiveness.
