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   Thermal Gas Systems Refrigerant Leak Detection and Monitoring Systems...

Tech Topics
Refrigerant Leak Detection and Monitoring Systems...


Refrigerant Gas Monitoring Technology

Photoacoustic Infrared is the most advance technology available for the detection of refrigerant gas. Only Photoacoustic Infrared (PIR) technology can provide a direct measurement of the refrigerant gas concentration that may be present in the mechanical room. To understand the advantages of Photoacoustic Infrared, it is important to first understand how each technology works.

Infrared technology is based on the scientific principle that specific gases absorb infrared light at specific wavelengths. This allows us to identify and quantify gas volume by measuring infrared light absorption. Specific gases can be selectively targeted by narrowly filtering the wavelength of the infrared light introduced; thereby avoiding any infrared absorption by other non-targeted gases that may be present in the sample.  While all infrared technology utilizes this principle, not all infrared systems measure gas concentration in the same way. 

NDIR (Non-Dispersive) or Absorptive IR systems sold by other manufacturers determine gas concentration by comparing an air sample from the machine room to a sample of inert gas (usually nitrogen) stored in the monitor. The room sample and reference gas are individually irradiated with infrared light. The light absorption measured in each gas sample is compared, and the difference between the two determines the concentration.  Inaccuracies with this technology can occur when the assumed zero baseline measurement of the reference gas drifts due to changes in room temperature and atmospheric pressure, aging of IR light source, or contamination of the reference cell.   

Photoacoustic IR (PIR) eliminates the problems associated with zero drift by eliminating the need to compare results to a reference sample.  Photoacoustic IR systems directly measure the changes in pressure that occur when infrared light is absorbed by the refrigerant present in the sample. The greater the pressure, the greater the concentration. Because no comparison to a reference gas is ever used, PIR does away with the possibility of  “zero drift.”  With Thermal Gas System Photoacoustic Infrared instruments, “zero-is-zero.”

CMOS: (ceramic metal oxide semiconductor) CMOS is an older but still useful technology for refrigerant leak detection.  CMOS technology is based on the change in conductivity of sensitized metal oxides upon exposure to refrigerant gases.  While CMOS systems perform adequately to satisfy the safety requirements of all refrigerants, IR technology is recommended for toxic applications, refrigerant conservation efforts, or in areas where cross contamination by extraneous chemicals is a factor.

Sensitivity of a refrigerant monitor is quantified in ppm, parts per million volume.  Both CMOS and Photoacoustic Infrared technologies can deliver adequate sensitivity to meet safety standards that call for an alarm, remote notification, and ventilation at the TLV-TWA (AEL-TWA).  The sensitivity of a highly selective IR instrument is 1 ppm; Wideband multi-gas IR is 20 ppm; CMOS instrument sensitivity is 30-40 ppm for HCFC's, 40-50 ppm for HFC's, and 60-80 ppm for CFC's.  To put these numbers in perspective, 1 lb. of refrigerant will evaporate to occupy 3-4 cubic feet of volume, thus raising the concentration of a 30,000-40,000-cubic-foot room to 100 ppm.

Selectivity is the ability of the instrument to differentiate between refrigerants.  When there is only one refrigerant present in the mechanical room and the likelihood of interfering vapors is remote, selectivity may not be an important issue.  This rationale may also apply when there are multiple refrigerants, but they are all of Safety Group A1.  When both Group A1, and non-Group A1 refrigerants are present in the same space (link to exposure limit table), or when other halocarbons, or volatile hydrocarbons may also be present, sensitivity can be very important.  Physical separation of potential leak sources can also decreases the importance of selectivity, even when two or more refrigerants are present.  For the highest level of selectivity, choose a Haloguard IR or II/IR unit.


System Design Considerations

Due to the wide variation in equipment room layouts, each situation must be considered individually. The following information is a general guideline for CMOS and Infrared systems.

Remote Notification
ASHRAE 15-2004 requires that audible and visual alarms be present inside the mechanical room as well as outside each entrance to the mechanical room.  System compatible remote strobe lights and audible alarms are available to meet these requirements.  The Haloguard Remote Display panel is also available to provide gas concentration and diagnostic information, visual and audible alarms outside the mechanical room.  All Haloguard Monitors can also communicate to Building Management Systems.

Number of Sensors/Sample Points
How many sensors do you need?  A good rule of thumb is that there should be one sensor or sample point for each 20,000-30,000 cubic feet of room volume, or no less than one sensor/sampling point fewer than the total number of chillers, whichever is less; provided that there is one sensor for each refrigerant safety group used in the room.

Location of Sensors/Sample Points
The ability of a monitor to measure the refrigerant concentration is dependent on the location of the sensing point. The sensing point may be remotely located up to several hundred feet from the controller. The controller and sensor/sensing point should be rigidly mounted indoors. The controller should be located in an area where the display can be viewed from most parts of the room and where it can be easily accessed for occasional calibration and service. The sensor/sampling point location should be approximately 18 inches above the floor in an area where refrigerant vapors are most likely to accumulate. Sensors/sampling points should be located in low-lying areas for occupant safety or near each potential leak source for if refrigerant conservation is a high priority.  

Airflow Patterns
If there is a continuous draft in the room a sensor/sampling point should be located downstream from the last potential leak source.  Airflow patterns can also cause areas of the room to become stagnant and allow refrigerant vapors to accumulate.  The sensor/sampling point location should be between the refrigerant leak source (chiller) and the ventilation exhaust.  Smoke tubes can be useful in determining the ventilation patterns.

Equipment Configuration
The equipment arrangement in the room can also affect the proper place to sample or locate a sensor. As a general guideline, if there is one chiller in the room, sample at the perimeter of the unit.  For two chillers, sample between them.  For three chillers, sample between each pair of chillers. (Note: we recommend using 2 or more points of monitoring). With four or more chillers, multiple monitors or a single monitor with a multipoint sampling system should be used.

Activities in the Room
The expected activity in the room, should be taken into account when choosing sampling locations. Some activities may require locating the sensing point above or below the 12 to 18 inch height. Traffic patterns can also affect airflow.   If unsure as to location, contact Thermal Gas Systems, Inc. to discuss.

Exposure Limits

GAS TLV-TWA1 STEL-C2 WEEL3 PEL4 RCL5 Group

R-11

·

1,000

·

1,000

1,100

A1

R-12

1,000

·

·

1,000

18,000

A1

R-22

1,000

·

·

1,000

25,000

A1

R-123

50**

·

50

30**

9,100**

B1

R-134a

1,000**

·

1000

1,000**

50,000**

A1

R-500

1,000

·

·

1,000**

29,000**

A1

R-502

1,000

·

·

1,000**

35,000**

A1

1) ACGIH (American Conference of Government Industrial Hygienists) - TLV-TWA = 8hr./day 40 hr./wk avg.
2) ACGIH - Short Term Exposure Limit Ceiling. Should not exceed 3-5 times the TWA for more than 30 min./day.
3) AIHA - Workplace Environment Exposure Level
4) OSHA - Permitted Exposure Limit
5) ASHRAE - Refrigerant Concentration Level for Occupied Space
**Exposure limits are pending. Given values were deterimined in a consistent manner.


DuPont Recommends Raising HCFC-123 AEL to 50ppm

The DuPont Acceptable Exposure Limit (AEL) Committee has recommended that the AEL of hydrochlorofluorocarbon (HCFC)-123 be raised to 50 parts per million (ppm). The AEL was set at 30 ppm in 1993. The new AEL is provisional but will become final in six months. More than five years of toxicology research, including recent studies conducted within the past two years, led to this recommendation.

In addition, the Workplace Environmental Exposure Limit (WEEL) for HCFC-123 was recently recommended at 50ppm. The WEEL is another measure of the potential safety of HCFC-123 and is the concentration to which nearly all workers can be exposed, day after day, for an entire working lifetime without experiencing adverse health effects. The WEEL was recommended by the Workplace Environmental Exposure Limit Guide Committee of the American Industrial Hygiene Association (AIHA).

AIHAs Workplace Environmental Exposure Committee is an independent, multi-discipline group that sets chemical exposure limits for the workplace. Toxicologists and industrial hygienists from industry, government regulators, and academics sit on the committee, including as representatives from the U.S. Environmental Protection Agency (EPA).

The significance of this is that the industry has come to closure on HCFC-123 with respect to occupational exposure. An independent group, as well as DuPont, has settle on an exposure limit, says Dr. William (Bill) Brock, toxicology manager of DuPont Fluoroproducts.

The EPAs Reva Rubenstein agrees. Our office is pleased to see consensus on a reasonable and appropriate long-term exposure limit for HCFC-123, she says. EPA believes this decision was made on the basis of high-quality data. It was the unusual quality and quantity of the research that allowed the WEEL committee to come to consensus so readily.

While these decisions are of more interest to chemical manufacturers, whose employees must concern themselves with possible day-to-day exposure, they also brought response from the HVAC industry.

The DuPont recommendation to raise the AEL of HCFC-123 to 50 ppm is a welcome development for users of large-tonnage chillers. HCFC-123 continues to be a key refrigerant, along with HFC-134a and HCFC-22, for liquid chillers used for air-conditioning in large buildings and for process applications, says Bill Dietrich, global product manager for large-tonnage chillers for York International Corp.

The WEEL is a long-term exposure limit. For the HVAC industry, if you are doing what you have to do by law ... recover the refrigerant ... the exposures are typically less than five minutes. This means short-term, not long-term, exposure is the issue, says Gene Smithart, environmental manager for the Trane Company. From a short-term exposure standpoint, we have known from early on that HCFC-123 is actually safer than the time-tested refrigerant, CFC-11, which it replaces.

HCFC-123 is marketed by DuPont as SUVA®(123 and is used globally as an alternative to chlorofluorocarbon CFC-11 in building air-conditioning systems.

DuPont offers a broad range of CFC alternatives for various applications: SUVA® (refrigerants), VERTREL® (specialty fluids), FORMACEL® (blowing agents), DYMEL® (aerosol propellants) and FE® (fire extinguishing agents).

July 1997

 

 

 

 

 

Choose Thermal Gas System's Photoacoustic IR for the highest level of monitoring dependability.

Because no reference gas comparisons are required, only Photoacoustic Infrared (PIR) technology provides a direct measurement of refrigerant gas concentrations. Other Non-Dispersive IR technologies are subject to “zero” baseline drift when comparing room samplings with a reference gas. PIR eliminates zero drift. With Thermal Gas System PIR instruments, “zero-is-zero.”
Proven, ultra-precise microphone enables unattended operation for longer periods of time.
PIR reduces installation and maintenance costs. No need for fresh air sampling line or in-line scrubber required by other IR units.
Higher operating efficiency. PIR technology eliminates downtime for frequent auto-zero processing common in other technologies.
Temperature controlled, sealed sample cell in Thermal Gas PIR units eliminates effects of temperature, atmospheric pressure and humidity changes on readings.
Superior signal-to-noise ratio. Highly selective PIR technology outperforms all others in trace measurment concentrations (1 ppm).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

All Haloguard Monitors Are Built To Order For The Least Cost Based On:

Monitoring Priorities:

Maximum refrigerant conservation
Occupant safety
Regulatory Compliance

Mechanical Room:

Size & layout
Number of entrances
Airflow patterns

Chillers:

How many?
What types?

Gas of Interest:

Virtually all refrigerants are supported
Number and Types of gases
Gas Combinations
Gas Combinations
Oxygen depletion monitoring is also available

Output Requirements:

Operate Local Devices – Relay contacts
Analog Output
Serial Communication
Data logging