International Space Station Environmental Control and Life Support System Overview

The Environmental Control and Life Support System (ECLSS) maintains a pressurized habitable environment, provides water recovery and storage, and provides fire detection and suppression within the International Space Station (ISS). This segment includes an overview of ECLSS and its component subsystems. It also describes the relations between subsystems, and between ECLSS and other Station systems.

6.2 Objectives

After completing this section, you should be able to:

· Describe the major functions provided by ECLSS and each of its subsystems

· Identify major ECLSS hardware components and state their function and general functional redundancies

· Identify major ECLSS functional dependencies on the Thermal Control System, Electrical Power System, and Command and Data Handling System

· Identify major functional support provided by ECLSS for the Thermal Control System, Payloads, Crew Health Care System, and Extravehicular Activity System

· Describe major responsibilities and milestones of the United States Orbital Segment (USOS) and Russian Orbital Segment (ROS) through 8A, as well as the USOS added capabilities at Assembly Complete.

6.3 ECLSS Overview

The Environmental Control and Life Support System (ECLSS) provides a pressurized and habitable environment within the Space Station by supplying correct amounts of oxygen and nitrogen, controlling the temperature and humidity, removing carbon dioxide and other atmospheric contaminants, and monitoring the atmosphere for the presence of combustion products, and major constituent proportions. The system also collects, processes, and stores water and waste used and produced by the crewmembers, and provides fire detection, suppression, and crew safety equipment.

The general functions of the five major subsystems of USOS ECLSS at Flight 8A are shown in Figure 6-1. As illustrated, the primary ECLSS concern, whether directly or indirectly, is with the ISS atmosphere. This section of the ISS Familiarization Manual presents ECLSS through its five subsystems: Atmosphere Control and Supply, Atmosphere Revitalization, Temperature and Humidity Control, Fire Detection and Suppression, and Water Recovery and Management.

Figure 6-1. ECLSS Subsystem interfaces at Flight 8A

6.3.1 Atmosphere Control and Supply

In Figure 6-2, the USOS Atmosphere Control and Supply Subsystem and its interfaces are illustrated for a Flight 8A configuration. This subsystem provides oxygen and nitrogen to maintain the Space Station atmosphere at the correct pressure and composition for human habitation. The Atmosphere Control and Supply Subsystem also provides gas support to various users on the Station, as well as pressure equalization and depressurization capabilities.

Figure 6-2, the ECLSS USOS Atmosphere Control and Supply Subsystem

The Russian Orbital Segment (ROS) has primary responsibility for atmosphere control and supply functions at the Flight 8A configuration. The Progress resupply vehicle is outfitted with tanks that can be filled with either nitrogen, air, or oxygen. These tanks are manually opened by the crew if the cabin pressure is low. Oxygen for the Station is primarily supplied by an oxygen generator called the Elektron, which electrolyzes water into hydrogen and oxygen. Additional oxygen can be provided by a Solid Fuel Oxygen Generator that uses chemical cartridges to produce oxygen in an exothermic reaction.

6.3.1.1 USOS Atmosphere Supply, Distribution, and Control

The oxygen and nitrogen gases used by the USOS Atmosphere Control and Supply Subsystem are provided through the supply, distribution, and control portions of the subsystem. Four highpressure gas tanks, two of nitrogen and two of oxygen, are stored on the exterior of the Airlock. The gases are distributed to the various users by a plumbed system running throughout the USOS. Another system of high-pressure plumbing allows the tanks to be recharged by the Shuttle. An oxygen compressor housed in the Airlock enables the oxygen tanks to be fully recharged because the Shuttle does not store oxygen at a high enough pressure to fully recharge the tanks. Empty tanks can also be replaced with full tanks as a second resupply option.

The Pressure Control Assembly monitors atmospheric pressures, controls the introduction of nitrogen and oxygen into the cabin atmosphere, and provides the means to depressurize Station volumes if required. The depressurization function is used in standard operations to relieve atmospheric overpressure, and in emergencies to vent hazardous contaminants overboard or as a last resort to extinguish a fire.

6.3.1.2 User Support

The Atmosphere Control and Supply Subsystem provides gas support to several users on the Station besides the atmosphere. Nitrogen is used to pressurize the Internal Thermal Control System accumulators and to calibrate the Crew Health Care System Volatile Organic Analyzer. The biggest users of nitrogen resources are the Payloads. A minor subsystem of ECLSS, the Vacuum System, is included here because its Payload support functions are similar to those of the Atmosphere Control and Supply Subsystem. The nitrogen provided to payload user via the vacuum system has vacuum resource and exhaust capabilities. Oxygen is provided for Extravehicular Activities (EVAs) and to the Fire Detection and Suppression Subsystem Portable Breathing Apparatus (PBA). The Airlock Depressurization Pump also supports normal EVA activities by pumping most of the Crew Lock air into Node 1 before the crew egresses.

6.3.1.3 Pressure Equalization Between Modules

The Atmosphere Control and Supply Subsystem also provides Manual Pressure Equalization Valves to equalize pressure between Space Station modules while the hatch is closed. A valve on each USOS hatch permits pressure equalization as modules are added to the Station, during normal EVA activities, or in the event that a module has been isolated in a contingency procedure. There are similar pressure equalization devices on the ROS hatches.

6.3.1.4 Additional Atmosphere Control and Supply Capabilities at Assembly Complete

At Assembly Complete, the Atmosphere Control and Supply Subsystem will have the addition of an oxygen generator on the USOS and the Sabatier on the ROS. The Sabatier conserves Station resources by producing water through reaction of hydrogen from the Elektron with carbon dioxide from the Atmosphere Revitalization Subsystem. At Assembly Complete, Atmosphere Control and Supply will also provide oxygen to the Water Recovery and Management Subsystem Potable Water Processor to assist in the purification of Station water.

6.3.2 Atmosphere Revitalization

Figure 6-3 shows the USOS Atmosphere Revitalization Subsystem and its interfaces. This subsystem ensures that the atmosphere provided by the Atmosphere Control and Supply Subsystem remains safe and pleasant to breathe. It performs carbon dioxide removal, trace contaminant control, and major atmospheric constituent monitoring.

Figure 6-3 ECLSS Functional Overview USOS Atmosphere Revitalization Subsystem and its Interfaces

6.3.2.1 Major Constituent Monitoring

The Major Constituent Analyzer monitors the composition of the Station atmosphere by mass spectrometry. Measurements are used to control the addition of oxygen and indirectly, nitrogen, into the Station atmosphere by the Atmosphere Control and Supply Subsystem, and to monitor the performance of the assembly that removes carbon dioxide. In the ROS, the Gas Analyzers use several different gas detection methods to provide similar functions. Air is delivered to the Major Constituent Analyzer by a network of pipes, valves, and sample ports running throughout the USOS. This network is the Sample Delivery System.

6.3.2.2 Carbon Dioxide Removal

The Carbon Dioxide Removal Assembly (CDRA) collects carbon dioxide from the cabin atmosphere with a series of regenerable sorbent beds and expels the unwanted gases to space. To remove carbon dioxide effectively, the CDRA requires cold, dry air, so it receives air from the Temperature and Humidity Control Subsystem and interfaces directly with the Internal Thermal Control System Low Temperature Loop. On the ROS, the Vozdukh performs the same function as the CDRA. Lithium hydroxide-based canisters are available for backup ROS functionality.

6.3.2.3 Trace Contaminant Control

The Trace Contaminant Control Subassembly filters and catalyzes numerous gaseous contaminants and odors from the cabin atmosphere. These contaminants are caused by material off-gassing, leaks, spills, or other events. On the ROS, the Trace Contaminant Control Unit operates similarly to the Trace Contaminant Control Subassembly. The Harmful Impurities Filter provides backup contaminant control as needed.

6.3.2.4 Additional Atmosphere Revitalization Capabilities at Assembly Complete

At Assembly Complete, the USOS adds duplicates of each of the Major Constituent Analyzer, Carbon Dioxide Removal Assembly (CDRA), and Trace Contaminant Control Subassembly. These duplicates will serve as backup only, because each device operates at a three-person rate. The ROS has similar capabilities which will be used in conjunction with the USOS equipment, and together they will be sufficient to support a six-member crew.

6.3.3 Temperature and Humidity Control

Figure 6-4 shows the USOS Temperature and Humidity Control Subsystem and its interfaces. This Subsystem helps maintain a habitable environment within the Station atmosphere by circulating cool dry air, removing humidity and particulates, and maintaining the temperature. Circulation of the atmosphere minimizes temperature variations, ensures homogeneous atmospheric composition, and provides a means for smoke detection. Three levels of circulation are provided: rack, intramodule, and intermodule ventilation. Rack ventilation cools and circulates air within an individual rack. Intramodule ventilation provides circulation to ensure a consistent atmosphere within a single module, and may support cooling and humidity removal. Finally, intermodule ventilation circulates air between modules to ensure a homogeneous atmosphere throughout the Station. While the ROS equivalent of Temperature and Humidity Control equipment is considered a part of the ROS Thermal Control System, it is functionally very similar to the USOS equipment.

Figure 6-4 ECLSS Functional Overview Temperature and Humidity Control Subsystem

6.3.3.1 Rack Ventilation

The Avionics Air Assembly is used to cool and circulate air within a specific rack volume. A fan and non-condensing heat exchanger provide cooling for rack equipment and circulation for operation of the Fire Detection and Suppression Subsystem Smoke Detectors. This heat exchanger interfaces with the Internal Thermal Control System Moderate Temperature Loop.

6.3.3.2 Intramodule Ventilation

The Common Cabin Air Assembly (CCAA) contains a fan and condensing heat exchanger to provide intramodule ventilation, temperature control, and humidity removal. Before the air enters the CCAA, it is drawn through the High Efficiency Particulate Air (HEPA) filters which remove particles and bacteria from the airstream. Moisture in the Station atmosphere is condensed by the heat exchanger and is sent to the Water Recovery and Management Subsystem. The heat exchanger interfaces with the Internal Thermal Control System Low Temperature Loop. Air that has been cooled and dehumidified is sent to the Atmosphere Revitalization Subsystem Carbon Dioxide Removal Assembly (CDRA) for its effective operation. There are two CCAAs in the U.S. Lab, of which only one is normally in operation at Flight 8A. There is also a CCAA in the Airlock which is only used during EVA operations. The Cabin Air Fan in Node 1 provides intramodule circulation but has no cooling or humidity removal capability.

6.3.3.3 Intermodule Ventilation

The Intermodule Ventilation (IMV) Assembly is a series of fans and valves that circulate air between modules through a ducting system. Hard plumbed ducts are located in most endcones on the USOS, and the hatches themselves can also act as circulation paths. The ROS relies on drag-through flexible ducting and open hatches for intermodule ventilation.

6.3.3.4

Additional Temperature and Humidity Control Capabilities at Assembly Complete

At Assembly Complete, additional Common Cabin Air Assemblies (CCAAs) will be available and Intermodule Ventilation (IMV) equipment will be included in all new modules. Additional Avionics Air Assemblies will be manifested as required to support rack equipment.

6.3.4 Fire Detection and Suppression

Figure 6-5 shows the USOS Fire Detection and Suppression Subsystem and its interfaces. The Fire Detection and Suppression Subsystem provides smoke detectors for the Station volumes, fire extinguishers, portable breathing equipment, and a system of alarms and automatic software responses for a fire event. Following a fire, the Atmosphere Revitalization and Temperature and Humidity Control Subsystems work together to remove contaminants from the affected volume. In an extreme situation, the Atmosphere Control and Supply equipment may be used to depressurize a module to extinguish the fire and/or exhaust the contaminants.

Figure 6-5, ECLSS Functional Overview Fire Detection and Suppression Subsystem

6.3.4.1 Smoke Detection

In the USOS, the Fire Detection and Suppression Subsystem provides two area Smoke Detectors in each pressurized module, and a Smoke Detector in each rack requiring an Avionics Air Assembly. These Smoke Detectors operate on a light obscuration principle, and are mounted in Temperature and Humidity Control Subsystem air paths. The ROS has two types of Smoke Detectors. One type is similar to USOS Smoke Detectors, and the other is an ionization type.

6.3.4.2 Fire Indication

A Caution and Warning (C&W) Panel mounted in each USOS module features lighted emergency buttons. If smoke is detected, flight software will light the “FIRE” button, sound an alarm, and shut off Temperature and Humidity Control equipment in the area to minimize oxygen being fed to the fire. Crewmembers may also sound (or silence) a fire alarm by manually pushing the button on the C&W Panel or on the Portable Computer System (PCS).

6.3.4.3 Fire Extinguishing

Fires on the USOS can be extinguished with handheld Portable Fire Extinguishers, which are filled with carbon dioxide. These function very similarly to typical fire extinguishers here on Earth. Two different nozzles allow the Portable Fire Extinguisher to be used on both open area and rack fires. The ROS uses fire extinguishers filled with a non-toxic nitrogen based substance that can be dispensed as a foam or a liquid.

6.3.4.4 Supplemental Oxygen Supply

During a fire, crewmembers must wear a Portable Breathing Apparatus (PBA), which is essentially a gas mask and an oxygen bottle. The PBA can also be plugged into oxygen ports provided by the Atmosphere Control and Supply Subsystem. The PBA is particularly important for a crewmember using the Portable Fire Extinguisher, because carbon dioxide displaces oxygen in the vicinity. This high concentration of carbon dioxide could cause the crewmember to lose consciousness if he or she is not supplied direct oxygen through a PBA.

6.3.4.5 Additional Fire Detection and Suppression Capabilities at Assembly Complete

There are no functional differences between 8A and Assembly Complete; there will simply be more equipment available.

6.3.5 Water Recovery and Management

In Figure 6-6, the USOS Water Recovery and Management Subsystem and its interfaces are illustrated for a Flight 8A configuration. This subsystem collects, stores, and distributes the Station's water resources. The water collected includes condensate from the Temperature and Humidity Control Subsystem and return water from EVA activities. At 8A, collected water is transported to the ROS for processing or is vented overboard.

Figure 6-6, ECLSS Functional Overview Water Recovery and Management Subsystem

6.3.5.1 ROS Water Recovery and Management Overview

The ROS has primary responsibility for water recovery and management functions for most of the assembly stages of the Station. The ROS collects condensate water from its condensing heat exchangers and receives water that is manually transported from the USOS. It is then purified and monitored for quality. If water is needed in the Elektron oxygen generator, potable water must be repurified to remove minerals. Small tanks are used to store and transport water in various locations on the ROS. Larger tanks with a pump assembly, called Rodniks, store potable water both on the exterior of the Service Module and on Progress modules. Solid waste products from various sources are collected and put on a Progress module for incineration upon atmosphere re-entry.

6.3.5.2 USOS Water Recovery and Management

At Flight 8A, the USOS collects condensate water from the Temperature and Humidity Control Subsystem and waste water from the Extravehicular Mobility Units (space suits). Waste water lines transport the water throughout the USOS, and the water is stored in a tank until it is removed from the system by overboard venting or manual transport to the ROS.

6.3.5.3 Additional Water Recovery and Management Capabilities at Assembly Complete

At Assembly Complete, the USOS Water Recovery and Management Subsystem has several more capabilities. A network of pipes and another tank will be used to transport and store water produced by the Shuttle’s fuel cells for use as make-up water on the ISS. A Urine Processor will separate water from urine and refine it to waste water. A Potable Water Processor will then refine waste water (including condensate, fuel cell water, EVA waste water, and Urine Processor output water) into potable water.

6.4 ECLSS Interfaces

In Figure 6-7, the interfaces that ECLSS shares with other ISS systems at Flight 8A are illustrated. At Assembly Complete, the USOS will also provide potable water to Crew Systems and to EVA Systems.

Figure 6-7. ECLSS interfaces at Flight 8A

6.5 ECLSS Milestones

There are several important milestones in the buildup of ISS ECLSS. Because ECLSS is primarily concerned with the maintenance of living conditions, these milestones correspond to the arrival of pressurized modules.

6.5.1 Flight 1A/R - Functional Cargo Block

The Functional Cargo Block (FGB) is the foundation module of the ISS. It contains the first few pieces of ECLSS equipment, including circulation fans and non-condensing atmospheric heat exchangers, fire detection and suppression equipment, and a Gas Analyzer.

6.5.2 Flight 2A - Node 1

At Flight 2A, Node 1 and two Pressurized Mating Adapters (PMAs) are delivered to the Station. Node 1 contains a Cabin Air Fan, Intermodule Ventilation equipment, a Cabin Pressure Sensor, Fire Detection and Suppression equipment, and Manual Pressure Equalization Valves on each hatch.

6.5.3 Flight 1R - Service Module

Most of the Russian ECLSS equipment on the Station at Flight 8A is housed in the Service Module. This includes the Elektron for oxygen generation, Vozdukh and lithium hydroxidebased canisters for carbon dioxide removal, a Trace Contaminant Control Unit and Harmful Impurities Filter for contaminant removal, Gas Analyzers for major constituent monitoring, fire detection and suppression equipment, air cooling and humidity removal equipment, urinal and commode facilities, and a condensate water processor. The arrival of the Service Module marks the beginning of the three-person permanent presence capability.

6.5.4 Flight 5A - Lab

Much of the USOS ECLSS equipment available during most of the assembly stages arrives with the Lab. Atmosphere Control and Supply equipment includes gas lines, Pressure Control Assembly, and Manual Pressure Equalization Valves. A complete rack of Atmosphere Revitalization equipment arrives, which contains the Major Constituent Analyzer, Carbon Dioxide Removal Assembly, and Trace Contaminant Control Subassembly. The Sample Delivery System lines launched in the Lab and Node 1 are connected to the Major Constituent Analyzer. Temperature and Humidity Control equipment includes two Common Cabin Air Assemblies and more Intermodule Ventilation equipment, as well as Avionics Air Assemblies in several racks. Fire Detection and Suppression equipment is launched with the Lab, as are the Water Recovery and Management condensate tank, Water Vent Assembly, and waste and (unused) fuel cell water lines.

6.5.5 Flight 7A - Airlock

At Flight 7A, the Airlock is installed, along with much of the Atmosphere Control and Supply storage and distribution equipment, more Manual Pressure Equalization Valves, and another Pressure Control Assembly. Another Common Cabin Air Assembly and more Intermodule Ventilation equipment arrive, and Sample Delivery System lines are connected to the USOS network. The standard Fire Detection and Suppression equipment is manifested, as is the Depressurization Pump. This flight marks the last major USOS ECLSS build-up until the arrival of Node 2.

6.6 Summary

6.6.1 ECLSS Purpose and Functions

6.6.2 Subsystem Names and Functions

There are five major subsystems within ECLSS:

· The Atmosphere Control and Supply Subsystem provides oxygen and nitrogen to maintain the Station atmosphere at the correct pressure and composition for human habitation. It also provides gas support to various users on the Station, and pressure equalization and depressurization capabilities

· The Atmosphere Revitalization Subsystem ensures that the atmosphere provided by the Atmosphere Control and Supply Subsystem remains safe and pleasant to breathe. It performs carbon dioxide removal, trace contaminant control, and major atmospheric constituent monitoring

· The Temperature and Humidity Control Subsystem helps maintain a habitable environment by circulating air, removing humidity and particulates, and maintaining the temperature of the Station atmosphere. Three levels of circulation are provided: rack, intramodule, and intermodule ventilation

· The Fire Detection and Suppression Subsystem provides smoke detection sensors for the Station volumes, fire extinguishers, portable breathing equipment, and a system of alarms and automatic software actions to annunciate and automatically respond to a fire event

· The Water Recovery and Management Subsystem collects, stores, and distributes the Station's water resources.

Figure 6-8 shows how all the components of the subsystems interact with each other.

6.6.3 Milestones

There are three major milestones in the buildup of USOS ECLSS capabilities. On Flight 2A, Node 1 contains the first pieces of USOS ECLSS equipment, consisting principally of ventilation and Fire Detection and Suppression equipment. Next, on Flight 5A, the Lab module carries a majority of the U.S. ECLSS equipment discussed in this manual, including major portions of all subsystems. Finally, on Flight 7A in the Airlock, the full capabilities of Atmosphere Control and Supply are enabled, and additional Temperature and Humidity Control and Fire Detection and Suppression equipment ensures crew health and comfort during EVA operations.