Exploring the Core: A Psychrometric Comparison of Liquid and DX Mechanical Systems

HVAC systems are vital in regulating indoor comfort and for maintaining indoor air quality. This psychrometric chart helps analyze how DX (Direct Expansion) mechanical systems and liquid desiccant systems reach desired temperature and humidity levels. If dehumidification is desirable or necessary, DX systems will cool the air to the dew point and below in order to remove humidity and then will be required to reheat it in order to reach to target supply air temperatures, while liquid desiccant systems adjust the air directly to the target state. This psychrometric chart above shows the energy each system needs to condition air from 95°F dry bulb/78°F wet bulb to 75°F dry bulb/62.5°F relative humidity.

However, the additional work required to reach these conditions comes at a cost. Due to the “overcooling then reheating” process, DX systems tend to be more energy-intensive and thus more expensive to operate over its lifetime Meanwhile, liquid desiccant systems, while potentially having incrementally higher capital costs in some cases, can be more cost-effective in the long run due to their enhanced dehumidification capabilities and lower energy consumption.

The psychrometric analysis comparing liquid and DX mechanical systems in HVAC involves studying the properties of moist air and the changes in these properties under various conditions. The following is the comparison of liquid systems and DX mechanical systems in HVAC:

System Overview

Liquid desiccant systems are designed to remove moisture directly from the air using a hygroscopic liquid solution, such as lithium chloride. They provide precise humidity control, making them ideal for industrial environments where strict moisture management is critical. The absorbed moisture is removed through a regeneration process, which allows the desiccant to be re-concentrated and reused.

Conversely, Direct Expansion (DX) systems cool and dehumidify air using refrigerants that absorb heat and moisture. These systems are typically used in smaller spaces — such as homes or small commercial buildings — where their simplicity and lower costs are key benefits. While less effective at precise humidity control than liquid desiccant systems, DX systems are valued for their manageable capital cost and ease of installation.

Humidity Control

Liquid desiccant (LD) systems can offer superior humidity control compared to DX systems, particularly in high-humidity load environments. These systems simultaneously lower the air temperature while stripping moisture from the air without requiring overcooling, and they can be easily integrated with reheat systems and other AHU’s to control temperature precisely and independently.

Meanwhile, humidity control is directly linked to the cooling cycle in DX systems. These systems must cool the air to lower temperatures (below the dewpoint) in order to get moisture to condense out of the air.. Unlike LD systems, substantial moisture removal requires sub-cooling, so substantial reheat is often required to meet supply air temperature targets.

Energy Efficiency

Liquid systems are generally more energy-efficient for large-scale applications. Centralized chillers can operate at higher efficiencies compared to multiple smaller DX units. These systems exhibit better part-load performance, as chillers can modulate their capacity and utilize thermal storage systems. Additionally, liquid systems can leverage free cooling methods, such as cooling towers or economizer modes.

DX systems can be energy-efficient for smaller applications and have reasonable operating costs. They can be easier to install and maintain, reducing operational costs for small-scale applications. However, DX systems can suffer from lower energy efficiency at part-load conditions since each unit operates independently.

Maintenance and Reliability

Liquid systems usually require a different set of maintenance skills, as do any typical hydronic systems such as chilled water air handling units (AHU’s), chiller systems, or even Combined Heat and Power (CHP) cogeneration facilities. These are often selected due to energy cost advantages or other economies of scale.While centralized systems mean a failure can have a wider impact, redundancies are often built in to mitigate this risk.

On the other hand, DX systems can sometimes be easier and less costly to maintain because of their simpler components and systems. Failures in DX systems can often be more localized, affecting smaller areas or zones, which can be advantageous in terms of operational continuity. As stated before, this simplicity often comes at an energy-use penalty or lack of economy of scale.

Practical Applications

Listed below are the applications where liquid systems excel, which generally include environments where the humidity load is high or where strict humidity control is required:

Dedicated Outdoor Air Systems (DOAS): excellent at removing moisture before entering a variety of workspaces, allowing existing HVAC equipment to work more efficiently.
Indoor Agriculture: great at managing the large humidity loads, while scrubbing the air of pathogens and providing energy savings.
Large commercial buildings: ideal for substantial cooling and humidity loads.
Hospitals, pharma, food, beverage and laboratories: excellent for strict humidity and temperature control to manage the unique nature of these applications.

Practical applications where DX systems are often used:

Residential buildings: preferred for lower initial cost and easy maintenance.
Small commercial spaces: suits spaces where precise humidity control is less critical.

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At AirGreen, we have revolutionized indoor climate control with our advanced HVAC systems, harnessing liquid and DX mechanical solutions for superior comfort. Our pioneering closed-loop liquid desiccant system is central to our technology, which not only cools the air but also effectively removes excess moisture. This dual-action approach ensures optimal air quality and comfort, making us a desirable and sustainable solution for improving indoor work conditions while reducing energy consumption.