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The purpose of this document is to show how a Fujitsu VRF system can be properly selected and designed to comply with ASHRAE Safety Standard 15 and Standard 34. ANSI/ASHRAE Standard 15 is widely recognized as the main guide for the safe use of refrigeration equipment. It is typically referred to in model codes as well as state and local codes. The Standard limits the allowed maximum refrigerant charge so that a complete discharge due to a leak into a small, occupied, and enclosed room can never exceed an allowable limit. As with most standards, ASHRAE Standard 15 is an application based standard, not an equipment design guide, so substantial engineering judgment might be required when designing a system using refrigerant. In addition, since Standard 15 does not specifically address VRF technology, it is good practice to review local codes and work closely with the local “authority having jurisdiction” (AHJ) when designing a system. All images and examples presented herein are conceptual and for explanation purposes only.
RCL=Total Refrigerant Charge [Allowable Refrigerant Concentration]×Total Occupied Volume
To calculate the Refrigerant Concentration Limit (RCL) for a multi-zone system according to ASHRAE 15 & 34:
- Identify the Refrigerant Type: Refer to the refrigerant's designation and safety classification.
- Determine Occupied Space Volume: Calculate the total volume of the spaces served by the system.
- Apply the RCL Formula: The standard typically defines the formula as:
- Account for Leak Scenarios: Ensure compliance with system leak detection and ventilation requirements.
Would you like a detailed step-by-step breakdown specific to the document's guidance?
To calculate the Refrigerant Concentration Limit (RCL) for a multi-zone system in compliance with ASHRAE Standards 15 and 34, follow these steps:
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Identify the Refrigerant's Practical Limit (PL): Consult ASHRAE Standard 34 to determine the Practical Limit (PL) for the specific refrigerant used. The PL represents the maximum allowable concentration of the refrigerant in an occupied space, typically expressed in pounds per 1,000 cubic feet (lb/1,000 ft³).
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Determine the Volume of the Smallest Occupied Zone: Calculate the volume of the smallest occupied space (V_smallest) that the refrigerant system serves. This is crucial because the smallest space dictates the maximum permissible refrigerant charge to prevent exceeding the PL in case of a leak.
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Calculate the Maximum Allowable Refrigerant Charge: Use the following formula to determine the maximum permissible refrigerant charge (M_max) for the system:
Mmax=PL×Vsmallest×11,000M_{\text{max}} = \text{PL} \times V_{\text{smallest}} \times \frac{1}{1,000}Mmax=PL×Vsmallest×1,0001
Where:
- MmaxM_{\text{max}}Mmax = Maximum allowable refrigerant charge (in pounds)
- PL = Practical Limit of the refrigerant (in lb/1,000 ft³)HVAC Talk+5ashrae.gr+5americangassafety.com+5
- VsmallestV_{\text{smallest}}Vsmallest = Volume of the smallest occupied space (in ft³)ashrae.gr
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Compare with the System's Total Refrigerant Charge: Determine the total refrigerant charge (M_system) of the entire system. Ensure that:
Msystem≤MmaxM_{\text{system}} \leq M_{\text{max}}Msystem≤Mmax
If the system's refrigerant charge exceeds MmaxM_{\text{max}}Mmax, design modifications are necessary to comply with safety standards.
Example Calculation:
Assume a system uses R-410A refrigerant with a PL of 0.26 lb/1,000 ft³, and the smallest occupied space has a volume of 1,000 ft³.ashrae.gr
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Calculate MmaxM_{\text{max}}Mmax:
Mmax=0.26 lb/1,000 ft3×1,000 ft3×11,000=0.26 lbM_{\text{max}} = 0.26 \, \text{lb/1,000 ft}³ \times 1,000 \, \text{ft}³ \times \frac{1}{1,000} = 0.26 \, \text{lb}Mmax=0.26lb/1,000 ft3×1,000ft3×1,0001=0.26lb
Therefore, the maximum allowable refrigerant charge is 0.26 pounds.
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Assess System Compliance:
If the system's total refrigerant charge is 0.25 pounds, it complies with the standard. If it is 0.27 pounds, modifications are required to reduce the charge or increase the smallest occupied space's volume.
Additional Considerations:
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Ventilation Impact: If the occupied space has a mechanical ventilation system operating during occupancy, the effect of air changes may be considered in calculating the effective volume, potentially allowing for a higher refrigerant charge. ashrae.gr
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System Classification: ASHRAE Standard 15 classifies refrigeration systems based on the likelihood of refrigerant leaking into occupied areas. High-probability systems, where leaks could directly enter occupied spaces, must adhere strictly to the calculated RCL.
By following these steps and considerations, you can ensure that your multi-zone system complies with ASHRAE Standards 15 and 34 regarding refrigerant concentration limits, thereby maintaining occupant safety and system integrity.
The common units of measure for refrigerant charge are:
Primary Units (Most Common)
- Pounds (lb) — Commonly used in the United States for HVAC systems.
- Ounces (oz) — Often used for smaller systems or when precise measurements are needed. (16 oz = 1 lb)
- Kilograms (kg) — Commonly used in metric systems and internationally.
Additional Units (Less Common in Field Applications)
- Grams (g) — For very small systems like mini-splits or appliance compressors. (1 kg = 1,000 g)
- Tons of Refrigerant (TR) — Used to describe larger system capacities rather than actual refrigerant charge.
Conversion Reference
- 1 lb = 0.4536 kg
- 1 oz = 28.35 g
When calculating refrigerant charge limits (like the RCL in ASHRAE 15 and 34), the charge is typically expressed in pounds (lb) or kilograms (kg).
