Glycol Chiller vs DX Cooling for Food Plants — Choosing by ASHRAE 90.1 / Thailand BEC

Thai food processing plants — from raw-material cold rooms to temperature-controlled production lines to blast freezers — face one foundational question that decides capital cost, safety and 15 years of electricity bills: choose DX (Direct Expansion) where refrigerant evaporates directly in the coil, or a Glycol Chiller secondary loop?

The answer is not "which is better" but which fits the job — number of zones, refrigerant type, food-safety strictness and energy goals. This article separates them by real criteria.

1. Two Architectures — Where Does the Refrigerant Sit?

The core difference is where the refrigerant evaporates.

flowchart LR
    subgraph DX["DX — Direct Expansion"]
        A1[Compressor] --> A2[Condenser]
        A2 --> A3[Expansion Valve]
        A3 --> A4["Evaporator coil
in room/production line
refrigerant evaporates here"]
        A4 --> A1
    end
    subgraph GLY["Glycol Chiller — Secondary Loop"]
        B1[Compressor] --> B2[Condenser]
        B2 --> B3[Expansion Valve]
        B3 --> B4["Chiller barrel
refrigerant evaporates here"]
        B4 --> B1
        B4 -.cools.-> C1["Propylene Glycol
20-40%"]
        C1 --> C2[Pump]
        C2 --> C3["multiple use points
(rooms / lines)"]
        C3 --> C1
    end

• DX: refrigerant pipes run straight to coils inside the cold room/production line and evaporate there — single circuit, no intermediary.
• Glycol: refrigerant evaporates at the central chiller, cooling propylene glycol, which is pumped to the use points — refrigerant never leaves the machine room.

2. Core Comparison Table

3. Food Safety — Never Swap PG for EG

In food work, the glycol you pick directly affects safety:

Hard rule for food work: any area that may contact product → PG only. Even though EG has better heat transfer and is cheaper, the food-safety risk is not worth it.

Choosing PG concentration depends on the lowest working temperature (freeze protection) — the freeze point must sit comfortably below the actual glycol temperature to avoid slush in the HX:

The more concentrated, the more viscous → higher pump head and pumping energy. Don't over-dose beyond need.

4. Energy — What ASHRAE 90.1 / BEC Say

ASHRAE 90.1 (and Thailand's Building Energy Code, administered by DEDE) set minimum chiller efficiency and load-control guidance:

• Minimum COP / IPLV — chillers must pass both full-load and part-load (IPLV per AHRI 550/590)
• Plants run mostly at part-load — so IPLV matters more than full-load COP for real energy estimates
• Economizer / free cooling — glycol systems can use free cooling via a dry cooler on cool nights (an advantage of the secondary loop)

The glycol penalty comes from two directions:

1. Approach temperature of the central HX → forces ~3-5°C lower evaporating temperature to deliver cold-enough glycol → COP drops
2. Pumping energy for viscous glycol, especially at high concentration

Together, glycol uses roughly 10-20% more energy than DX at the same load — the price paid for safety and flexibility.

5. The Refrigerant Angle — Why This Matters More Each Year

In the Kigali phase-down era, the architecture choice is tied directly to refrigerant type and charge:

• Mildly flammable A2L refrigerants (R-32, R-454B, R-1234ze) and A3 (R-290) have a low-GWP future, but ASHRAE 15 / EN 378 cap charge-per-area strictly
• A glycol secondary loop solves this directly — all refrigerant is held at the chiller in a ventilated, sensor-equipped machine room, while the production area sees only safe glycol
• DX that runs refrigerant piping into production space is charge-limited → limits size/run length, or forces higher-GWP A1 refrigerants

This is why many new food plants choose secondary glycol + low-GWP refrigerant at the chiller as the future-proof path. The refrigerant that charges the chiller (e.g. R-449A, R-513A, R-454B) and the propylene glycol are what must be sourced and topped up across the system's life.

6. How to Match the System to the Job

flowchart TD
    Q1{Multiple zones/lines
needing separate control?} -->|Yes| GLY[Lean Glycol]
    Q1 -->|No, single point| Q2{A2L/A3 refrigerant
or large charge?}
    Q2 -->|Yes| GLY
    Q2 -->|No, A1 small charge| Q3{Need max COP
+ low first cost?}
    Q3 -->|Yes| DX[Choose DX]
    Q3 -->|Need very stable temp| GLY
    GLY --> R1[Use PG in food zones
+ free cooling if feasible]
    DX --> R2[Manage charge per ASHRAE 15
+ pick refrigerant for the space]

Choose DX when:

• Single point / single zone, first cost is the constraint
• You want maximum COP at the design load
• Using an A1 refrigerant with a charge small enough to pass ASHRAE 15

Choose a Glycol Chiller when:

• Multiple zones/lines needing separate, precise temperature control
• Using A2L/A3 refrigerant or a large charge → must confine refrigerant to the plant room
• You need high temperature stability (swing-sensitive product) or plan free cooling
• You want to future-proof against low-GWP refrigerants

Conclusion

DX wins on first cost and single-load COP; the Glycol Chiller wins on refrigerant-charge safety, temperature stability, multi-zone control and future-proofing with low-GWP refrigerants — at the cost of 10-20% higher energy and higher capital.

For new Thai food plants designed around ASHRAE 90.1 / BEC and planning for refrigerant phase-down, a secondary glycol system with propylene glycol often pays off long-term. But for a single steady-load point, DX remains the most direct and economical path.

Whichever you choose, both need quality refrigerant and ongoing top-ups — the Sahawatthanakit team is ready to advise on matching refrigerant selection to your system architecture and project TOR.