This air conditioning unit is specifically designed and manufactured for roof-mounted installation on locomotives. At World Part Supplier, we take pride in the global reach of our products, which are widely used across international markets with a strong record of customer satisfaction and performance reliability.

A Range of Cooling and HVAC Solutions for Locomotives

At World Part Supplier, we manufacture and supply a diverse line of HVAC systems for locomotive applications, tailored to meet a variety of operational needs:

1. Roof-Mounted Air Conditioning Units (Cooling Only) – Designed for reliable performance in high-temperature environments.
2. Roof-Mounted HVAC Units (Cooling & Heating) – A dual-function system offering year-round climate control inside the cabin.
3. Under-Cabin Air Conditioning Systems – Installed below the locomotive cabin to optimize space while ensuring efficient air distribution.

These categories reflect our commitment to designing purpose-built, high-performance systems that deliver long-term reliability and comfort, no matter the route or climate.

 Figure 1. 3D View of WPS-0005000 Air Cooler

Our engineering team incorporates tropical climate specifications into the design of our HVAC systems by applying the principles of fluid mechanics and referencing the psychrometric chart. This approach ensures that each system is optimized for performance in high-humidity, high-temperature environments.

Given the diversity of operational environments—such as locomotives running on mainline routes versus those operating in mining areas—certain default settings may need to be adjusted based on geographic location and usage conditions. To accommodate this, the air conditioning unit’s smart control system includes an administrative interface that allows the purchasing company’s technical team to customize performance parameters, ensuring optimal functionality in every application.

Psychrometric Chart:
A psychrometric chart is a graphical representation of the physical and thermodynamic properties of moist air, such as dry-bulb temperature, wet-bulb temperature, humidity (relative and absolute), enthalpy, and air density. It is primarily used to understand how these variables interact and to interpret occupant comfort, design strategies, and energy requirements in various applications, particularly in HVAC.

The applications of psychrometric charts span a diverse range of fields:

• HVAC (Heating, Ventilation, and Air Conditioning): They are essential for designing, optimizing, analyzing, and troubleshooting air conditioning systems, playing a critical role in ensuring optimal indoor air quality and thermal comfort.  

• Thermal Comfort and Building Design: The charts are used to interpret occupant comfort and develop effective passive design strategies that minimize reliance on mechanical systems. 

• Meteorology: They assist in understanding weather patterns and predicting atmospheric conditions. 

• Industrial Processes: Psychrometric principles find application in various industrial operations, including drying, cooling towers, and humidification, where precise control of air-water vapor mixtures is critical.

  Figure 2. Psychrometric Chart

What are the parts of the Psychrometric Chart?

There are eight standard parts in the Psychrometric chart:
1. Dry Bulb Temperature (DBT): The dry bulb temperature is the most commonly understood measure of air temperature, obtained using a standard thermometer with a dry sensing bulb. On the psychrometric chart, dry bulb temperature is represented by vertical lines that run parallel to the humidity ratio axis. As one moves from left to right along the horizontal axis, the air temperature increases.

2. Wet Bulb Temperature (WBT): The wet bulb temperature is defined as the lowest temperature to which an air mixture can be cooled solely through the process of evaporative cooling, which involves the addition of water. It is measured by a thermometer whose sensing bulb is covered by a wet wick. As water evaporates from the wick, it absorbs latent heat, thereby lowering the temperature of the bulb. The extent of this cooling effect is inversely proportional to the air humidity; drier air facilitates more evaporation and thus results in a lower wet bulb reading. Notably, when the air reaches 100% relative humidity (saturation), there is no further evaporation from the wick, causing the wet bulb temperature to equal the dry bulb temperature.  

Graphically, wet bulb temperatures are indicated by diagonal lines that slope downward and to the right across the chart, originating from the saturation curve and extending towards the bottom. These lines are often close to enthalpy lines, though they are not perfectly parallel.

3. Relative Humidity (RH): Relative humidity is a critical indicator of the moisture content of the air. It is expressed as a percentage and represents the ratio of the actual amount of water vapor present in the air to the maximum amount of water vapor that the air could hold at the same temperature and pressure. Essentially, it quantifies the air moisture content relative to its maximum moisture-holding capacity at a given temperature. 

On the psychrometric chart, relative humidity is depicted by curved lines that sweep from the lower left to the upper right. The topmost, leftmost curve represents 100% relative humidity, known as the saturation curve, with values decreasing as the lines move downward and to the right. These lines are commonly indicated in intervals of ten percent.

4. Humidity Ratio (Absolute Humidity/Specific Humidity): The humidity ratio, also frequently referred to as moisture content or mixing ratio, quantifies the mass of water vapor present per unit mass of dry air. It essentially represents the actual weight of water vapor in a given amount of dry air.  

The units for humidity ratio are typically expressed in grains of moisture per pound of dry air, or grams of moisture per kilogram of dry air. On the chart, the humidity ratio is represented by horizontal lines that originate from the right vertical axis. This vertical axis itself is dedicated to displaying the humidity ratio values. 

It is important to clarify the distinction with "specific humidity." While sometimes used interchangeably with humidity ratio, specific humidity is more precisely defined as the mass of water vapor as a proportion of the total moist air sample (including both dry air and water vapor). This means its value is always slightly lower than the humidity ratio. Standard psychrometric charts typically plot the humidity ratio (mass of water vapor per mass of dry air) on the vertical axis.

5. Dew Point Temperature (DPT): The dew point temperature is the critical temperature at which water vapor in the air will begin to condense into liquid water or frost if the air is cooled at a constant pressure and humidity ratio. This phenomenon occurs precisely when the air reaches 100% saturation.  

On the psychrometric chart, dew point temperature is represented by horizontal lines that extend from a given state point to the saturation curve (the 100% RH line). The actual dew point temperature reading is then found along the saturation curve or on a dedicated scale located on the right side of the chart. A key characteristic of dew point temperature is that it remains constant as the dry bulb temperature varies, changing only with the absolute moisture content of the air.

6. Enthalpy: Enthalpy serves as a measure of the total energy contained within a thermodynamic system, specifically the humid gas (the air-water vapor mixture). It quantifies the sum of both sensible heat (heat that changes temperature) and latent heat (heat associated with phase changes, like evaporation or condensation). This property is crucial for understanding how much "stored" energy will be consumed or released when the air temperature or moisture content is altered.  

Enthalpy is commonly expressed in units such as Joules per kilogram (J/kg) or kilojoules per kilogram (kJ/kg), or in British Thermal Units per pound (BTU/lb) of dry air. On the chart, lines of constant enthalpy slope downward and to the right, appearing nearly parallel to the wet bulb lines. Values for enthalpy are typically read from scales positioned on the extreme edges of the chart, often along a diagonal axis. Enthalpy is vital for calculating the total heat added or removed during air conditioning processes, which is essential for determining cooling loads, heating requirements, and ventilation needs

7. Specific Volume: Specific volume represents the volume occupied by a unit mass of dry air, including its associated water vapor. It is mathematically defined as the reciprocal of density (mass per unit volume). 

The units for specific volume are typically expressed in cubic meters per kilogram (m³/kg) or cubic feet per pound (ft³/lb) of dry air. On the psychrometric chart, lines of specific volume are sloped, generally running from the top-left to the bottom-right. Some chart formats may show them originating on the horizontal axis and sloping upwards and to the left. The specific volume serves as a measure of the air density and is influenced by both its temperature and moisture content. It is crucial for determining the mass or weight of air when the volume is known, a critical step for calculating the heat or moisture requirements in HVAC systems.

8. Other Graphical Elements: 

8.1. Saturation Curve (100% RH Line): This prominent line is the leftmost curved boundary on the chart. Along this line, the air is fully saturated with water vapor, meaning its relative humidity is 100%, irrespective of temperature. At any point on this curve, the dry bulb, wet bulb, and dew point temperatures are all equal. The specific graphical orientation of each psychrometric property, such as vertical lines for dry bulb temperature, horizontal lines for humidity ratio and dew point, and curved or diagonal lines for relative humidity, wet bulb temperature, and enthalpy, is not arbitrary. This arrangement directly reflects the thermodynamic relationships between these properties. For example, dew point lines are horizontal because dew point is primarily a function of the absolute moisture content (humidity ratio), which is represented on the vertical axis, and is largely independent of dry-bulb temperature unless saturation is reached. Similarly, constant dry-bulb lines are vertical because dry-bulb temperature is the primary x-axis variable. This inherent graphical logic ensures that the chart is intuitively readable once the fundamental relationships are understood. The convergence of dry-bulb, wet-bulb, and dew-point temperatures at the saturation curve (100% RH) signifies a critical thermodynamic state where the air can hold no more moisture. This point is crucial for predicting condensation, which is a significant concern in building design due to the potential for mold growth and decreased insulation performance. Furthermore, this convergence is also relevant in agricultural applications for understanding and implementing frost protection strategies. This convergence is not merely an interesting thermodynamic fact; it is a direct indicator of the onset of a phase change (condensation), which has profound practical implications for building durability, indoor air quality, and agricultural viability. Engineers and designers utilize this information to prevent moisture-related damage and optimize environmental control.

8.2. Data Points: These individual markers on the chart represent the specific air conditions at a given time and location. They can be plotted to reflect hourly, daily, monthly, or even seasonal data. The density and distribution of these data points on the chart can provide insights into average conditions over a period.

8.3. Comfort Zone: Often overlaid on the chart, the comfort zone is an area that delineates the range of thermal conditions within which building occupants are generally satisfied. By plotting actual air conditions relative to this zone, designers can assess occupant comfort and explore how passive design strategies might expand this comfortable range.

8.4. Polygons: Some psychrometric charts incorporate polygons to group data points that share common climate properties. These polygons aid in the selection of beneficial design strategies by visually categorizing climate types and their associated challenges. 

HVAC System Design, Analysis, and Troubleshooting

Psychrometric charts are indispensable for the comprehensive lifecycle of HVAC systems. In the design phase, they assist engineers in determining the precise cooling and heating requirements for buildings, enabling the creation of systems tailored to specific climatic conditions. They allow for the prediction of the leaving supply air temperature of a system and facilitate the design of units optimized for diverse climates. For analysis, the charts provide a clear understanding of air patterns, how various air properties interact, and their collective impact on overall system performance. They are used to analyze and maintain comfortable temperature and humidity levels for occupants within conditioned spaces. In troubleshooting, by plotting the changes in air properties as air moves through an air handling system, the chart becomes a valuable diagnostic aid for identifying and resolving air temperature and humidity problems, as well as issues within automatic HVAC control systems.

With the above considerations in mind, let’s now take a closer look at the operational logic of our flagship roof-mounted locomotive air conditioning unit, model number WPS-0005000. This base model represents the foundation of our HVAC lineup and reflects the engineering expertise and field-tested reliability that World Part Supplier is known for.

Figure 3. Logic of Operation of the Air Conditioner Electrical System

The system is engineered to operate on a DC input voltage ranging from 60 to 85 volts. Maintaining the correct polarity of the incoming power supply is critical to the safe operation of the air conditioning unit. To prevent damage from incorrect wiring, a protective circuit has been integrated into the power input path. In the event of a reversed polarity connection, the circuit will automatically block the power from reaching the unit, ensuring the system remains unharmed.

When the incoming power supply falls within the defined operational range, it is routed into the air conditioner inverter system. The inverter consists of two proprietary subsystems:

1.  Fan Inverter – Controls both the condenser blower fan and the evaporator fan(s).

2.  Compressor Inverter – Exclusively manages the operation of compressors, ensuring efficient cooling cycles.

The system logic is tied directly to the selector switch, which features five distinct operational modes. When the user rotates the switch toward a cooling setting, the system receives a signal to activate both inverter branches. Conversely, if the selector is turned toward fan-only operation, only the fan inverter is engaged, leaving the compressor idle.

Selector Switch Positions:

1.  OFF – System completely shut down

2.  Low-Speed Cooling – Activates fans and compressor at lower capacity

3.  High-Speed Cooling – Full operation for rapid cooling and airflow

4 . Low-Speed Fan Only – Air circulation without active cooling

5 . High-Speed Fan Only – Maximum airflow with no compressor activity

Cooling Mode Performance Settings

When the selector switch is set to Cooling Mode, the user can choose between two operational levels:

1.  Low-Speed Cooling: In this setting, the inverter reduces power output to the evaporator fan by approximately 70%. This results in gentler airflow through the evaporator coil and delivers a lower cooling capacity, ideal for milder conditions or reduced energy consumption.

2.  High-Speed Cooling: In this mode, the system operates at full capacity, with the fan and compressor both running at 100% power. This delivers maximum cooling performance, ensuring fast cabin temperature reduction in hot or demanding environments.

Smart Startup Logic and Sensor-Based Control

Once the selector switch is turned to a cooling mode—regardless of whether it is set to low or high speed—the inverter initiates a self-diagnostic routine during the initial seconds to verify its internal health and system readiness. If no faults are detected and the control logic confirms safe operation, the inverter begins processing data from the following input sources:

1.  Ambient air temperature sensor (external)

2.  Cabin interior air temperature sensor

3.  Inverter temperature sensor

4.  High-pressure refrigerant circuit sensor

5.  Low-pressure refrigerant circuit sensor

The incoming sensor data is cross-referenced with the default thresholds programmed into the microprocessor of the system. If conditions permit normal operation, the inverter supplies power to the condenser blower fan.

Because this air conditioning system is engineered for extreme environmental conditions, the condenser fan will run for an initial three-minute pre-cooling phase. During this phase, it intensifies gas compression in the condenser unit to stabilize high-side refrigerant pressure and prevent compressor overload.

After this critical pressure regulation cycle, the compressor is activated with a soft-start mechanism, ensuring smooth and safe engagement with minimal electrical and mechanical stress.

 Figure 4. 3-Phase Inverter Set for Air Conditioner Purposes

To download the catalog for the WPS-0005000 locomotive air conditioning unit, please click this link.

If you are interested in placing an order for this product or have any questions, please contact World Part Supplier via the company’s official email address (info@WPSupplier.com) or visit our Contact Page.