An important operational target for all refrigeration systems can generally be said to be that they should be as tight and leak-proof as possible. There is no absolute leak-free system and the use of gas detection products aims to minimise and control this leakage. Leaking refrigeration systems generally pose a challenge to the refrigeration sector and require resolute action. Otherwise the industry may lose the initiative and be subjected to even tougher, mandatory laws and taxes to address, among other things, the associated negative environmental consequences.
This article is aimed at everyone who is involved in various ways with planning, installing and maintaining commercial and industrial refrigeration systems. It offers the user a comprehensive overview of what is required for different applications and how the existing standards are implemented. Ultimately, it is about personnel safety, profitability and the environment.
Who need a Refrigerant Monitoring System?
Every refrigeration machine room and every refrigerated room needs a refrigerant monitor, especially, if the system to be monitored contains more than 23 kg of refrigerant. There are several good reasons to install a gas monitoring system.
The main reasons are personnel safety and environmental concerns in line with current legislation. It is also financially profitable to have reliable facilities with as leak-free systems as possible.
There are three main reasons to install refrigerant monitoring systems:
- For personnel health & safety
- For environmental care
- For financial reasons
For personnel health & safety
Several of the gases in refrigeration plants are dangerous to humans. In lesser concentrations, ammonia can cause irritation to the respiratory tract and eyes, and in higher concentrations may lead to severe injuries and eventually death.
HFC (HCFC) refrigerants and carbon dioxide displace oxygen from the air and may ultimately bring about suffocation. HCs such as propane and isobutane are a hydrocarbon compound, containing only carbon and hydrogen. These compounds cause no damage to the environment, but are flammable. So, measures to minimise risk need to be taken. HCs may also cause suffocation.
For environmental care
Most of the gases in refrigeration plants have adverse environmental effects. The so-called F gases, the fluorinated greenhouse gases, are discussed the most. Some older gases with chlorine compounds are now totally banned within the EC and may only be used in recycled form.
HCFC
HCFCs like HCFC 123 and HCFC 22 are halogenated hydrocarbon compounds with chlorine that affect ozone depletion. These refrigerants are governed by Montreal Protocol and under phase out schedule in India.
HFC
HFCs like HFC 134a and HFC 410A are halogenated compounds that do not contain chlorine and thus do not affect the ozone layer. However, HFCs have a significant impact on global warming so emissions must be minimised. The uses of HFCs are regulated by the Kyoto Protocol and recently being included in Montreal protocol as well. It thus is in our common interest to keep refrigerants in systems that are as free of leaks as possible.
For financial Reasons
The refrigeration systems are, especially, prone to leaks: Vibration expansion and contraction in lines, breaks in capillary tubes, poorly connected flare connections all contribute to repeat service calls and early compressor failure. Statistically every refrigeration system loses its complete charge three times in ten years.
There is a direct relationship between refrigerant loss and compressor failure in a refrigeration system where failure can be traced back to inadequate refrigerant gas due to leaks.
Approximately 50% of supermarkets electric cost services the refrigeration system. And slow leak will gradually reduce a compressors capacity, and directly affect efficiency, causing the compressor to run more frequent and longer run times. This will directly reduce compressor life.
Refrigerant blends (400 Series) can leak the component with the highest saturation pressure first, leaving the blend in the system short of the chemical. When fresh refrigerant is added, blend is not the same. If a system is repeatedly charged, the resulting new blend will decrease equipment efficiency to the point that the entire charge.
Choosing the Right System
A gas detection system consists of a chain – from discovery of the risk to the corrective action. It is important to think through the measures to be taken at each level of alert, and to plan for the appropriate staff to be informed, such as the plant manager and maintenance contractor.
- What is the purpose of the alarm?
- Which gas(es) are to be detected?
- What detection principles are the most appropriate? How many sensors are needed, where and how should they be placed?
- What rules and regulations apply for the refrigerant in use?
- What is the refrigerant’s density relative to air?
- How does the ventilation affect the detected area?
- What steps are to be taken when an alarm occurs?
The function of different alarm levels
Alarms can typically be divided into Level 1, 2 and 3 alarms, also named low level alarm, main level alarm and high level alarm. Each different alarm level calls for different measures:
Normal (0 to 10 PPM): This PPM range is based on normal background readings of most refrigeration and comfort air applications. This level is considered safe for equipment and AEL recommendations
Low Level (25 PPM): Alarm for maintenance staff, this indicates possible leak. Repeated low alarms indicate a probable leak that should be identified and repaired for conservation purposes. The purpose of low level Alarm is to indicate that a possible leak exists and to provide early warning before the leak grows.
Main Level (50 PPM): Urgent alarm for maintenance staff, flashing light activated. This indicates a significant and growing leak. The probable leak should be identified and repaired as soon as practical, if not immediately. The purpose of main alarm is to indicate that a substantial or growing leak exists and minimize the refrigerant loss before it reaches a point which might affect personal and building operations. The building ventilations should not be started as this would hamper the ability to find leak.
High Level (150 PPM for R123 and 500 PPM for others): Emergency alarm as the main level alarm, with siren activated and alarm sent to the rescue services, refrigeration plant is shut down (power supply as well). This indicates that significant leak or possibly a catastrophic spill has occurred.
For R123, the level has exceeded the short term exposure level (STEL) of 150 PPM. The high speed exhaust ventilations should be turned on to reduce the level of below acceptable level exposure level of 50 PPM.
For other refrigerants, the level may have approached the AEL of 1000ppm. High speed exhaust ventilation should be turned on to reduce the level to below 50 ppm.
The purpose of high alarm is to indicate a major leak. In addition to ventilation, a remote area supervisor should be notified that a possible large leak has occurred and proper personal protective equipment might be needed in order to enter the space.
Typical Sample Location
Purpose of the alarm
To choose the right equipment and systems, a number of parameters first must be determined. This then controls the choice of products, their placement and the alert levels.
The main objectives of a gas alarm systems are:
- Leakage alarm, monitoring unoccupied space
- Emergency alarm
- Personnel health & security
- Warning of fire and explosion hazard
- Protect stored products.
Leakage Alarm, Monitoring Unmanned Space
Monitoring of accidental leaks in order to avoid downtime, protect the environment and minimise the loss of refrigerant. There are no established alarm limits since the need has to be adapted to each refrigeration plant. Practical experience shows that the alert levels based on sanitary limits usually are too low for effective leakage alarms. The sanitary limits in turn are different depending on the type of gas.
Emergency Alarm
Emergency alarm concerns in principle only ammonia plants and explosive gases. Emergency alarms will start the evacuation of buildings, neighbourhoods, etc and involve the monitoring of high concentrations directly dangerous to life and health. National legislation typically implies that a risk assessment or risk analysis must be made in all plants with ammonia. An action plan must be drawn up that, among other things, concerns how the staff should be alerted and act in order to be safe. In larger facilities, where there is a risk of external leakage to the surroundings, there must also be a plan for how the public should be warned.
Room Volume Considerations
Normal industry practice has been to think about refrigerant leaks in terms of grams of refrigerant per unit time, such as lbs/hr or gr/yr. This is a natural and logical way of looking at it. The system monitors the amount of refrigerant present in the air in parts per million (ppm) by volume or refrigerant molecules as compared to air molecules. In order to develop a correlation between the leak rate in weight per unit time and parts per million, there are a number of items that need to be considered. These are:
1 Room volume
2 The weight of refrigerant per unit volume at ambient temperature and pressure
3 The amount of time the refrigerant has been leaking
4 The rate at which fresh air enters the room and existing air is exhausted.
5 The location of the inlet, relative to the leak, air flow in the room and the rate at which the refrigerant expands to fill the room.
For a specific application, items 1 through 4 can be calculated, or estimated. Item 5 is virtually unpredictable, therefore in all calculations, it is assumed that the leaking refrigerant will expand to fill the room with an even distribution of refrigerant. This assumption will yield safer, conservative calculations. If the monitor sample and reference locations have been appropriately chosen, the monitor will see a higher concentration than calculated from the ideal formulas.
Formula Definitions:
ppm – refrigerant concentration
LR – Leak rate in cubic meter/hour
FA – Fresh air into the room in cubicmeter/hour
VOL – Room volume in cubic meter
t – Time in hours
R – Volume of refrigerant in cubicmeter
LRmin – Minimum leak rate that will result in a given ppm
RD – Refrigerant density in kg/cubicmeter
To be able to convert between ppm and leak rate in cubic meter/hour, the refrigerant density must be known, the refrigerant density of common refrigerants is
R-22 RD22 = 3.59 kg/cubic meter
R-123 RD123 = 6.56 kg/cubic meter
R-134a RD134 = 4.34 kg/cubic meter
Lets try with two examples to demonstrate the conversion. Case I is for a sealed room, with no air turnover. Case II is for a room with a known air turnover.
Case I: Sealed room 15 m x 10 m x 3 mhigh
- How much R-22 (in kg) is necessary to cause a measurement of 25 ppm?
Given 25 ppm R-22 in the sealed room:
Volume of R-22 = parts per million/million x room volume
R(cubic meter) = [25(ppm)/1,000,000] x [15 x 10 x 3](cubic meter)
R(cum) = .01125 cubic meter of R-22 in the sealed room
Given .01125 cubic meter R-22, calculate weight in kg:
Weight = volume (cubic meter) x density (kg/cubic meter)
Weight = R(cubic meter) x RD22(kg/cubic meter)
Weight = .01125(cubic meter) x 3.59(kg/cubic meter)
Weight = .040 kg
- If the leak rate is 150 kg/yr, how long will it take to reach 25 ppm?
Given the same refrigerant in the same room, 25 ppm weighs .040 kg:
Time (hr) = weight of R-22(kg) / [weight(kg)/time(yr)] x[time(hr)/time(yr)]
Time(hr) = .040(kg) / [150(kg)/1(yr)] x [8760(hr)/1(yr)]
Time(hr) = 2.34 hrs
Case II: 10 m x 15 m x 3 m high room with fresh air makeup of 225 cubic meter/hr
- How much R-22 (in kg/yr) is necessary to cause a measurement of 25 ppm?
Given air turnover of 225cubic meter/hr, calculate the leak rate that is required to maintain a measurement of 25 ppm:
Leak rate min (cubic meter/hr) = parts per million/million x fresh volume (cubic meter/hr)
Leak rate min (cubic meter/hr) = [25(ppm)/1.000.000] x 225(cubicmeter/hr)
Leak rate min (cubic meter/hr) = .005625 cubic meter/hr
Given .005625 cubic meter/hr, calculate the leak rate in kg/yr needed to reach 25 ppm:
Leak rate (kg/yr) = leak rate (cubic meter/hr) x density(kg/cubic meter) x [8760(hr) / 1(yr)]
Leak rate (kg/yr) = .005625(cubic meter/hr) x 3.59(kg/cubic meter)x 8760(hr/yr)
Leak rate (kg/yr) = 176.9 kg/yr, therefore the leak rate must be greater than 177 kg/yr.
- If the leak rate is 300 kg/yr, how long will it take to reach 25 ppm?
Calculate LR in cubic meter/hr:
LR (cubic meter/hr) = [leak rate(kg/yr) / density(kg/cubic meter)] /8760(hr/yr)
LR (cubic meter/yr) = 300(kg/yr) / 3.59(kg/cubic meter) /8760(hr/yr)
LR (cubic meter/yr) = .009539 cubic meter/hr
Calculate time in hr:
Time (hr) = [Room volume(cubic meter)/Air flow(cubic meter/hr)] xln[LR(cubic mete/hr) / [LR(cubic meter/hr) – Leak ratemin(cubicmeter/hr)]]
Time(hr) = [450(cubic meter)/225(cubic meter/hr)] x ln[.009539(cubic meter/hr)/[.009539(cubic meter/hr)- 0.005625(cubic meter/hr)]
Time(hr) = 2(hr) x ln[2.437] Time(hr) = 1.78 hr
Conclusion
Refrigerant monitors system offers two important benefits: refrigerant conservation and safety. With the decreasing availability of refrigerants, coupled with rising cost, it is important to have a refrigerant monitor in the equipment room to provide early warning of refrigerant loss. As a leak-sensing device, a refrigerant monitor constantly measures the amount of specific refrigerants in the surrounding air. It’s capable of initiating alarms, activating building ventilating systems, and integrating with building automation systems. Refrigerant monitors are also strongly recommended for all existing chiller rooms or refrigerant storage facilities.
AUTHORS CREDIT & PHOTOGRAPH
Kapil Singhal
Founder
B P Refcool
