Oye Hoye Chips, Lahore — humidity-controlled potato cold store with low-TD evaporator psychrometrics and seasonal CO₂ management

Oye Hoye is not just a brand — it is a supply chain problem solved in cold. Pakistan has two potato harvest windows: spring (March to May, predominantly Punjab) and autumn (September to November, Punjab and KPK). A chip manufacturer that produces year-round from a seasonal raw material faces an inventory equation that only cold storage can solve: buy at harvest — when supply peaks, prices bottom, and quality is highest — and store for up to eight months to feed continuous production.

The cold store is not a utility. It is the production scheduling infrastructure. Without it, United Snacks would be forced to buy from secondary markets in off-harvest months at 30 to 60% price premiums, accept variable-quality potatoes that have been stored poorly elsewhere in the supply chain, and risk production disruption when spot-market supply tightens. The cold store is the asset that converts a seasonal raw material into a year-round supply. Izhar Foster built the engineering that makes that asset work.

The psychrometric challenge — why standard evaporators fail in potato storage

Most cold stores run standard evaporators selected for thermal efficiency: a large temperature difference (ΔT) between the coil surface and the room air, typically 8 to 12°C. At a +7°C room setpoint, this means a coil surface running at −2 to −5°C. The coil acts as a dehumidifier — it is far below the room's dew point and strips moisture aggressively.

For a potato store at 85 to 90% relative humidity, this is a disaster. The room air at +7°C and 88% RH has a dew point of approximately +5.2°C. A coil surface at −3°C is 8 degrees below that dew point — it will condense moisture out of the room at a rate that overwhelms any practical humidifier. The result is potatoes at 60 to 70% RH, losing 1.5 to 2% of their weight per month through skin transpiration, developing soft spots, and beginning to shrink before they ever reach the fryer.

The solution requires psychrometric chart analysis at the evaporator selection stage. The design process begins with the psychrometric chart — plotting the room condition (+7°C, 88% RH), locating the dew point (+5.2°C), and selecting an evaporator whose coil surface temperature will operate above, or just barely at, that dew point. A low-TD evaporator achieves this by using a larger coil surface area — typically 40 to 60% more coil than a standard unit — so the required cooling capacity is delivered with a coil-to-room ΔT of only 3 to 4°C. The coil surface runs at +3 to +4°C instead of −3°C. It is cooling the room effectively without condensing its humidity.

The humidification system — compensating for the unavoidable

Even a well-selected low-TD evaporator removes some moisture — both through the inevitable latent component of its cooling effect and through the defrost cycles that run periodically to clear frost from the coil. Humidifiers compensate for this moisture removal and hold the room at 85 to 90% RH continuously.

The Izhar Foster installation uses ultrasonic disc humidifiers — devices that vibrate a piezoelectric disc at ultrasonic frequency to atomise water into a fine mist, which is then distributed by a fan into the room airstream. Ultrasonic units are preferred over evaporative wetted-pad types for high-load, precise-humidity applications because they can be modulated in fine steps, have no direct heating requirement, and do not require a water temperature high enough to sterilise legionella (the water droplet size of 1 to 5 microns is below the range where Legionella can proliferate in the airstream).

The BMS manages humidifier output against a capacitive humidity sensor mounted at the mid-height, mid-room position — away from evaporator discharge air and humidifier supply air, to measure the actual room average rather than either extreme. The control loop runs a proportional-integral algorithm: when humidity falls below 84%, the humidifier ramps to maximum output; as it approaches 90%, output modulates down to trim. The loop is interlocked with the evaporator defrost cycle — humidifiers pause during defrost and resume at a high ramp rate afterward to recover the humidity quickly.

CO₂ management — not a CA store, but not passive either

Potatoes left in a sealed room exhale CO₂ continuously as they respire. Unlike a controlled-atmosphere (CA) apple store — which actively injects CO₂ to a precise concentration to slow the apple's ripening — a potato cold store is managed in the opposite direction: remove accumulated CO₂ before it reaches a level that stresses the tubers or produces off-flavours in the processed chip.

The CO₂ vent-purge system works as follows:

• Sensors — NDIR (non-dispersive infrared) CO₂ sensors are mounted at the upper zone of the cold store, where buoyancy causes CO₂-rich air to stratify. Sensors sample air continuously and transmit readings to the BMS.
• Setpoint logic — When CO₂ exceeds the setpoint (typically 3,000 to 4,000 ppm depending on variety and intended storage duration), the BMS activates the vent-purge sequence.
• Dampers — Motorised supply and exhaust dampers open. Fresh outdoor air is drawn through a pre-cooling coil (to avoid thermal shock to the product) and distributed at low velocity through the store. Stale, CO₂-rich air is exhausted through the opposing damper.
• Duration and recovery — The purge runs until CO₂ drops to a lower threshold, then dampers close. The refrigeration and humidity systems recover the room to setpoint, which the BMS monitors and confirms before the next production pick.

The purge cycle also flushes ethylene — another potato respiration by-product — which accumulates slowly and can interact with sprouting inhibition chemistry in the tuber.

Seasonal swing — the design problem unique to Punjab chip manufacturers

A cold store for a Punjab chip manufacturer must work across one of the largest seasonal refrigeration load swings of any single-temperature application in Pakistan:

June: The spring harvest arrives at the store warm (+18 to +22°C from field). Lahore ambient is peaking at 44 to 47°C. The refrigeration plant must simultaneously pull down freshly loaded product and reject enormous heat to the condenser under the worst ambient of the year. This is the design peak case — the plant is sized for this moment.

December: The autumn harvest has arrived; the spring-harvest stock has been processed out. Lahore ambient is 10 to 15°C. The refrigeration load in the store is a fraction of summer — primarily product respiration heat and the small residual conduction gain through the PIR envelope at a 5 to 8 K gradient rather than the summer's 40 K gradient. A fixed-capacity system would short-cycle destructively.

January–February: Lahore ambient drops to +5 to +8°C. The room approaches thermal equilibrium with the outside. If ambient falls below the room setpoint, the outside is now trying to cool the room below +6°C — triggering low-temperature sweetening in the stored product. The system must switch from cooling to supplementary heating.

Izhar Foster addresses the full seasonal swing with:

• Variable-speed (inverter-driven) compressors — allow the plant to run at 20 to 100% capacity continuously, matching load without short-cycling. Inverter compressors also deliver the highest COP at part load, reducing electricity consumption significantly during the long low-load winter period.
• Supplementary electric heaters in the air distribution plenum, thermostatically controlled by the BMS, activated when room temperature approaches the +6°C lower limit. These run for brief periods in the coldest Lahore winter nights and are sized for the thermal balance calculation at −2°C ambient (ASHRAE 99% design), not oversized.
• Condenser capacity modulation — in winter, when the condenser has excess rejection capacity relative to the reduced compressor load, the BMS cycles condenser fans off in sequence to maintain a minimum condensing pressure and prevent the refrigerant circuit from operating outside its design pressure envelope.

The FireSafe PIR envelope — first line of thermal defence

Everything in the refrigeration system is working against heat that enters through the building envelope. In a +7°C store on a 47°C Lahore day, the thermal gradient driving conduction heat gain through the walls and roof is 40 K. Every watt of heat that enters through the envelope is a watt the refrigeration system must reject — and rejected heat costs electricity.

Izhar Foster's FireSafe PIR sandwich panels at 150 mm thickness achieve a U-value of 0.14 W/m²K with λ = 0.022 W/m·K (BS EN 14509, aged). At 40 K gradient, a 150 mm PIR wall conducts approximately 5.6 W/m². A typical EPS wall at the same nominal thickness conducts approximately 9.1 W/m² — 62% more. For a large potato store, that difference is measured in tens of kilowatts of refrigeration capacity and thousands of kWh per year in electricity.

The floor is an insulated-slab design with PIR board below the concrete, vapour barrier, reinforced screed, and hardened surface coating rated for forklift wheel loads from fully loaded potato bins. Floor edge insulation prevents thermal bridging at the perimeter — a common failure point in cold stores built to lower specification.

Parameter	Design value
Room setpoint	+6 to +8 °C (±0.5 °C control)
Relative humidity	85–90 % RH
CO₂ purge setpoint	3,000–4,000 ppm (NDIR sensors)
Evaporator type	Low-TD, ΔT 3–4 K, coil surface +3 to +4 °C
Humidifier type	Ultrasonic disc, modulating output
Compressor type	Inverter-driven, 20–100% capacity modulation
Seasonal heating	Electric element, BMS-activated below +6.5 °C
Condenser design ambient	47 °C (ASHRAE 0.4% + 2K, Lahore)
Panel thickness	150 mm PIR (U 0.14 W/m²K)
Floor insulation	75 mm PIR below slab
Storage duration	Up to 8 months post-harvest

Supply chain economics — the case for owned cold storage

A chip manufacturer has two options for raw-potato inventory: buy from spot markets year-round, or build a cold store and buy at harvest. The economics are decisive in Pakistan's market conditions.

Punjab potato prices at spring harvest typically range from PKR 30 to 50 per kg. Prices in December and January, during the supply gap between harvest windows, can reach PKR 90 to 150 per kg — a 2 to 4x premium. For a company processing hundreds of tonnes per month, the difference between harvest pricing and gap pricing is a capital expenditure that could justify a purpose-built cold store in two to three crop cycles.

Beyond price, quality is controllable in owned cold storage in a way it is not in third-party stores or spot market purchase. The variety, harvest date, field condition, and field-cooling treatment can be verified at intake. The storage temperature log can be queried for the full storage duration. If a batch of potatoes arrives with a known temperature excursion, that batch can be prioritised for earliest use — first-in-first-out inventory management by exception.

This is the case that United Snacks made when specifying this cold store, and it is the case Izhar Foster helps food manufacturers build at the initial scoping stage. Start with the heat load calculator to size the refrigeration, use the cost estimator for a budgetary number, then talk to engineers who understand what the product needs at every stage of the process.